Gen 4 added more cross-generation evolutions than any other generation. 29 new final forms for older Pokemon. Electivire, Magmortar, Rhyperior, and Togekiss all debuted here. New methods include leveling while knowing a specific move and leveling near magnetic or mossy/icy rocks.
Gen 4 added 29 new evolutions for existing Pokemon, more than any other generation. Electivire, Magmortar, Rhyperior, Togekiss, Honchkrow, Mismagius, Weavile, Gliscor, Roserade, Dusknoir, Gallade, Froslass, Tangrowth, Yanmega, Lickilicky, Ambipom, Mamoswine, Magnezone, and Probopass were all new final forms for older Pokemon.
Certain Pokemon evolve when leveled up near a special magnetic, mossy, or icy location. Magneton becomes Magnezone near Mt. Coronet's magnetic field. Eevee becomes Leafeon near the mossy rock in Eterna Forest, or Glaceon near the icy rock on Route 217. You just need to be in the area when the level-up happens.
Several Pokemon evolve by leveling up while knowing a specific move. Lickitung needs Rollout to become Lickilicky. Tangela needs AncientPower. Piloswine needs AncientPower too (for Mamoswine). Aipom needs Double Hit. Yanma needs AncientPower. If the Pokemon hasn't learned the move yet, visit the Move Reminder.
Electirizer (for Electivire) and Magmarizer (for Magmortar) are held by wild Elekid and Magby respectively, but Elekid is Pearl-exclusive and Magby is Diamond-exclusive. Dubious Disc, Protector, and Razor Fang drop from various sources. The Underground is your best bet for finding many of these items through digging.
Competitively, Togekiss (from Togepi line), Weavile (from Sneasel), and Gliscor (from Gligar) became top-tier threats. Garchomp was a brand-new pseudo-legendary. Mamoswine gave Piloswine a third stage with great Ice/Ground typing. The Gen 4 cross-gen evolutions transformed a lot of previously mediocre Pokemon into real threats.