Your caster level for the chosen spellcasting class increases by +4. This can't increase your caster level beyond your HD. However, even if you can't benefit from the full bonus immediately, if you later gain noncaster-level HD you may be able to apply the rest of the bonus.
For example, a human 5th-level cleric/3rd-level fighter who selects this feat would increase his cleric caster level from 5th to 8th (since Cleric+Fighter has a 8 HD). If he later gained a fighter level, he would gain the remainder of the bonus and his cleric caster level would become 9th (since Cleric+Fighter now has 9 HD).
A character with two or more spellcasting classes (such as a bard/sorcerer or a ranger/druid) must choose which class gains the feat's effect. This does not affect your spells per day or spells known. It only increases your caster level, which would help you penetrate SR and increase the duration and other effects of your spells. You may select this feat multiple times. Each time you choose it, you must apply it to a different spellcasting class.
For instance, a 4th-level cleric/5th-level wizard who had selected this feat twice would cast cleric spells as an 8th-level caster and wizard spells as a 9th-level caster.
The Practiced Spellcaster feat will increase the caster level by as much as +4 levels but is only effective for multi-classed spell casters. Paladins and rangers are fully affected by this feat. This means that a paladin 10 / sorcerer 6 with the Practiced spellcaster affecting his paladin levels will cast spells as a 9th level caster (paladin 10 = 5th level + 4 levels from the feat). Actually, the bonus is applied as soon as the ranger becomes multiclassed, it doesn't matter how many levels he has in the second class, for example, a ranger 11/blackguard 3 will have an effective casterlevel of 9 (11/2+4).
Effect on Crafting: The Practiced Spellcaster feat does count toward minimum caster level requirements for magical crafting. For example, a wizard 5 / fighter 1 / eldritch knight 1 with the feats Practiced Spellcaster and Craft Magical Arms and Armor may successfully use a magic workbench to enhance crafted armor with a +2 enhancement bonus. Adding the +2 enhancement bonus requires Caster Level 6, which the player would not have had without the Practiced Spellcaster feat.
Effect on Turn Undead: It is unknown whether the Practiced Spellcaster feat has any effect on the turn undead ability.
Bug Notes Edit
Contrary to the description, the extent of this feat is not capped by the character's hit dice, but by the number of levels in additional classes. Normally, the use of this feat operates as described above. However, if used in conjunction with a prestige class that grants additional spellcaster levels, this feat STILL grants up to +4 caster levels and does not cap at the character's HD as it should. Thus, a Wizard 10 / Eldritch Knight 10 that is using this feat will, in fact, have a caster level equal to 23 (Wizard 10 + Eldritch Knight levels 2 through 10 + this feat 4). A Wizard 10 / Eldritch Knight 2 would have a a caster level of 13 (10 from Wizard, 1 from Eldritch Knight, and 2 from the feat).
- Note: The extra caster levels gained by this feat contribute to the character's total caster level and have the potential to raise DC's at higher levels. For instance, the above Wizard 10 / Eldritch Knight 10 would benefit from the +1 Bonus to DC's for having reached caster level 23.
As of Patch 1.04 Edit
Currently, Practiced Spellcaster is a selectable feat during character level up.