Pros:
4 is smoother and more tactical, also headshots give you camouflages.
5 has better large-scale battles, the Toss Back perk, and some good maps.
Cons:
4 makes throwing back grenades almost useless, noob tubes (grenade launchers) are available at rank 1, pretty small maps.
5 is choppier, MP40s are way too powerful and accurate, most new perks are useless (flak jacket, shades, gas mask), MOTHERFUCKING TANKS, no weapon camouflage, lots of glitches/sploits.
Note: I'm only talking about multiplayer here.