Core Four
The Core Four system replaces normal class advancement with only three classes: Caster, Rogue, Seer, Warrior. These classes replace all other classes. There's no multiclassing.
This system also removes all class features and replaces them with feats. Even casters aren't given spells by the class, they select feats that grant them spells.
At every level you gain one or two feats chosen from your class list of feats. You gain one class feat at odd levels and two at even levels.
At odd levels you also gain a feat that can be selected from any list, provided you meet the prerequisites. Your effective level in all classes other than your own is half your actual level. So a Caster wanting to take an 3rd-tier Warrior feat would need to be at least 12th level because a warrior needs to be at least 6th level to gain a 3rd-tier feat and the caster's effective warrior level is half their caster level. This allows for flexible feat selection but limits options outside your class' scope.
Core Four feats don't synthesize well with games that use other classes and typically we don't recommend using them in such games. Core Four powers, however, can work well in any d20 system functioning in place of a spell of the same tier.