What's Different?There's many differences between each generation, so learning a different generation is like relearning Pokemon altogether. What exactly is different between IV and V?
Gen V Pokemon
Of course, you will never find a Pokemon from Gen V in Gen IV. There's no fear of Terrakion or Volcarona, no frustration about Ferrothorn, nothing about Landorus in sand or Tornadus and Keldeo in rain. With all these big, powerful threats absent, it's easier to make a team without worrying about too many specific threats.
Rotom's alternate forms each have a secondary typing based on its signature move. Wash ends up benefiting the most because it has great bulk typing and only one weakness. The others are in lower tiers and only Cut seems to actually be a threat in its own tier.
For Gen IV, every form is Electric/Ghost. With the exact same base stats, they are essentially the same Pokemon with a different move and look. Wash and Heat remain the top two while each has its own merits below them. While they lack STAB on their signature moves, they do gain the ability to use Substitute very well, being able to set up on Blissey with a SubCharge set. They are arguably the top OU spinblockers of Gen IV.
In Gen V ubers, Arceus is a devastating force, being able to run literally any EV spread it wants, any type of set it wants, and any type it wants. It holds many spots in Ubers usage due to its multiple types, and its usefulness in each type.
In Gen IV, however, Arceus does have a limit. In Gen IV, you are unable to EV train Lv. 100s, being limited to using the vitamins to the 100 EV mark. Arceus is only available at Lv. 100. That means Arceus must limit itself to 100 EVs in every stat but one, which will have 10. While this may sound annoying, Arceus remains terrifying, because it keeps bulk and power together with base 120 in all stats.
Many Pokemon banned in Gen IV aren't banned in Gen V, and Blaziken isn't banned in Gen IV (No Speed Boost). Latios, Latias, and Salamence were much too powerful for OU and were banned one at a time, as their main checks were each other. Wobbuffet and Wynaut were banned due to Shadow Tag being deemed broken. All Deoxys formes are banned in Gen IV, including Defense. If you're planning to make a team, remember not to use any of these.
The arguably most dominant force of Gen V is weather. Gen V brought Drought and Drizzle into OU, and because of the sudden opposition of weather, Sand Streamers became more common, and Abomasnow gained some usage as well. So many bans were imposed due to weather itself not being broken, but the Pokemon abusing it.
Gen IV only has Sand Stream and Snow Warning for permanent weather in OU, and neither has enough decent abusers to dominate the game. Sand is the most common, and even then it can be defeated by non-weather easily. Neither weather can weaken move types, fortunately.
Dream World brought Hidden Abilities to the table. Quite a few Pokemon remained the same as always, others had worse, but many improved greatly. Dragonite became the top dragon of OU with Multiscale, improving its already impressive bulk. Gliscor became the featured physical wall with Poison Heal, and with BW2, now has Roost, Stealth Rock, AND Poison Heal. Espeon jumped from NU to OU within months, all thanks to Magic Bounce, a permanent Magic Coat. Venusaur became the first Grass-type starter to reach OU, gaining Chlorophyll and Growth's new mechanics, and a partner in Ninetales. Alakazam rose to OU with Magic Guard, making it a fantastic sweeper with immunity to any damage that isn't an attack. Many more rose to fame with their newfound abilities.
Since this is a mechanic introduced with Gen V, Gen IV has zero of these. Dragonite and Gliscor are still in Gen IV OU, however both see considerably less use. Espeon dwells in the low ranks of NU, and Alakazam is stuck in UU. Venusaur is in UU because it still has reasonable bulk and decent typing for it.
Sturdy in BW(2) acts as a permanent Focus Sash. This makes Skarmory and Aron impossible to OHKO without Stealth Rock. Sturdy is very useful since it lasts throughout the entire battle, and with Roost/Wish support, Skarmory can constantly abuse it.
In Gen IV, Sturdy is useless by Smogon's standard rules. It merely blocks the opponent from using Horn Drill, Fissure, Guillotine, or Sheer Cold, which are all banned anyway. Skarmory's better off using the almost-just-as-useless Keen Eye.
Lightningrod and Storm Drain
Lightningrod and Storm Drain draw in and grant the holder immunity to Electric and Water-type moves, respectively. In addition, the user gains +1 Sp. Atk. Only a handful get each ability, and a majority don't care for the immunity and/or boost. Gastrodon enjoys it for being able to absolutely wall Rotom-W.
In Gen IV, neither ability gives an immunity or boost. They're rather useless in Single battles, leaving Gastrodon in low tiers.
Volt Switch is one of the best moves introduced in BW. It not only gave special attackers their own version of U-Turn, it also made the strategy of U-Turning constantly more viable, with Volt Switch added into it. Forretress also gained a way to escape Magnezone without using either EQ on the switch or holding Shed Shell.
Volt Switch does not exist in DPPt or HGSS. U-Turn is the only move to have this function in that generation. Although it has a wide distribution, many U-Turners are either physically based, or the special attackers holding it do not like giving up a special move for it. Mixed attackers also prefer to break walls as opposed to switch out.
Scald is a controversial move in Gen V. Now many more Pokemon can burn opposing Pokemon with a moderately powerful attack. Even better, Fire-types still don't like this move, even though they're immune to the burn.
The move is absent from Gen IV, which is a relief for players. Their non-weak physical attackers can now switch into Water-types worry-free.
Psyshock and Psystrike are in the same group because they did essentially the same thing: give special Psychic-types a good way to hit Blissey. Psystrike is Mewtwo's signature move, which got rid of its usability with Selfdestruct (plus the nerf).
Being absent in Gen IV, that means CM users struggle against Blissey. Mewtwo is forced to use Selfdestruct on its special sets if it plans to beat Blissey (and who wants to waste a perfectly good sweeper?).
Explosion and Selfdestruct
These two moves seem pretty normal. They kill the user and inflict damage to the opponent, with high base power. The problem is that they're Normal-type, and due to the lack of Pokemon with either STAB or physical power with it, they aren't devastatingly powerful, especially when resisted.
What sets these apart is the effect in Gen IV and earlier: they halve Defense. By doing this, they have a devastating amount of power, STAB or not. Pokemon that may resist it may not actually be able to take it very well.
ExtremeSpeed in Gen V has +2 priority. This means it beats all other increased priority attacks sitting at +1. Because of this, Lucario can use Swords Dance and still beat an Infernape with priority because ExtremeSpeed beats both Mach Punch and Vacuum Wave. Not only that, but it also beats Prankster users before they try to status or set up Substitutes (unless it's a Sableye). Dragonite also gains ExtremeSpeed but doesn't often use it outside the Mixed set and the Banded set.
For Gen IV, ExtremeSpeed is stuck at +1 priority. It's in the same boat as every other increased priority move on that level, so Infernape beats Lucario setting up.
Drain Punch and Giga Drain
Drain Punch and Giga Drain are essentially the same except for type. Each has 16 PP and base 75 power. Bulk Up Gallade and Breloom love Drain Punch when it comes to being a tank. Roserade likes Giga Drain for its bulky sets.
Unfortunately, both of these moves sit at base 60 power in Gen IV. Drain Punch has 8 PP maximum as well, which is why it's uncommon. If only both moves kept their power in Gen V, as Breloom and Roserade would enjoy the Technician boost.
Hi Jump Kick
Hi Jump Kick is an extremely powerful move, sitting at 90% accuracy and base 130 power, as well as the majority of users getting STAB. Its downside is that if it fails to strike the target, the user takes a massive amount of recoil. It does get boosted by Reckless, fortunately.
Before Gen V, HJK had the same effects, but it had only base 100 power. Close Combat outclassed it with a reliable accuracy and more power.
Eviolite is an item that flipped a few tiers on their heads. Porygon2 and Dusclops each quickly rose above their evolutions. Chansey rose to be equal with Blissey, as Blissey was still reasonably bulky and kept Leftovers. Many others adore this boost, especially down in LC or NFE.
Gen IV's lack of this item leaves the abusers in lower tiers. Porygon2 and Dusclops remain reasonably bulky on both sides, but just don't fit in OU very well. Pretty much anything else using the boost is NFE. LC is also even more offensively oriented due to the lack of bulk.
Air Balloon is one of the most infuriating items for some. It allows Heatran, one of the bulkiest offensive Pokemon, to avoid its greatest weakness, Ground. Air Balloon killed a few lure sets from Gen IV, such as offensive Forretress. It's all too easy to miss the message stating its use and accidentally EQ a floating Heatran.
The item doesn't exist in Gen IV. This makes it much easier to lure and kill Heatran, or just to trap it in general.
For Gen V, the sleep counter resets every time you switch out. This makes it difficult to make a Pokemon wake up once it's put to sleep. While there's Sleep Talk, Sleep Talk will fail on the turn the Pokemon wakes up. While Rest does give a definite time it will wake up, you aren't able to use Rest's healing while you're asleep, which makes it difficult to keep ResTalk going.
In Gen IV, you can constantly burn sleep turns and switch out when it's dangerous. Gen IV has a lot of ResTalk due to its usefulness.
Team Preview and Leads
Gen V has Team Preview, which is called out for being one of the worst implemented mechanics Game Freak introduced, but is actually one of the best. Team Preview allows both players to view each other's team and select their own team in any order. Many players in Gen IV liked surprising their opponent but this could not be done as well with Gen V.
With that said, Gen IV's metagame has a whole lot to do with leads. Your lead's match up against the opponent's can sometimes decide the whole game. Pokemon such as Aerodactyl can force switches with Taunt and set up their own Stealth Rock, while Machamp provides an immediate problem by blowing a hole in the lead or whatever switches in. Prediction is much more difficult at the start in Gen IV because you don't know the opponent's team yet. This is probably the fun part about Gen IV for people: unpredictability.
ConclusionNow that you're aware of all these major differences, you should be ready to make your own Gen IV team and enjoy the metagame!