in a nutshell, 4 was you pick a type of "base," offensive, defensive, or special teams. you have a certain amount of "points" based on XP. you "buy" units with those points, and can never have more than a certain amount at any time. and when your units die, those points are freed up and you just "buy" the unit again. the same applies for the enemy. and if you manage to take out their MCV type thing? they just get a new one.
but at least they learned from this and generals 2 is back to the standard CnC gameplay.
.... oh wait. browser game and micro transactions. yeah, that's going to be fun.