In the manual it says 'Your caster level for the chosen class increase by +4. This can't increased your caster level beyond your HD', which means a Wizard Lv20 gets nothing through it, while a Fighter 5/Wizard 20 can have a caster level at 24 instead of 20.
But I have a question: A Wizard 20/Archmage 5 has caster level of 25, or even 30 at most (Spell Power V). According to the explanation, it seems Practiced Spellcaster will do nothing on this occasion, for the caster level is already no less than HD. Am I right? Or Wizard 20+Practiced Spellcaster(Wizard)+Archmage 5+Spell Power V will rise the caster level to 34? Or this depends on the order I choose those feats?
Thanx~