I'm not sure that the feat breaks any particular d20 or D&D v.3.x paradigm, but it is certainly at the "broken" end of the feat usefulness spectrum. With one feat, a multi-class spellcaster enhances all caster level dependent elements of his or her spells by up to 4 levels. A heck of a lot of spell parameters are level dependent. Range, damage, SR penetration, bonuses granted etc. etc. This is an astounding increase in power at low to mid-levels, and a reasonable increase at higher levels. A Wiz1/Ftr4 would be casting his magic missiles as a Wiz5; a Clr1/Ftr4 would be casting his cure light wounds as a Clr5. (OK, the PC wouldn't get the feat 'til 6th level, but you get the point).