A multiclassed character with 4 sorcerer class levels and 2 cleric levels has an equal caster level for sorcerer spells as a single-class character with only 4 sorcerer levels but no cleric levels.
Generally, caster level is equal to the character's class level for the class which is providing access to that spell. If a character has access to the same spell from different classes only the specific class being used for preparing and casting each instance is used as the caster level.
Paladins and rangersEdit
The Practiced Spellcaster feat will increase the caster level by as much as +4 levels but is only effective for multi-classed spell casters and/or classes with levels that do not grant caster level. Paladins and rangers are fully affected by this feat as they are treated as half level for casting. 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) while a Paladin 10 will be caster level 9 as well (5 from paladin and 4 from practiced caster).
The Arcane Scholar of Candlekeep, Arcane Trickster, Eldritch Knight, Harper Agent, Pale Master, and Red Wizard of Thay prestige classes will increase caster level progression for arcane casters, while the Sacred Fist, Stormlord, Doomguide and Harper Agent will increase caster level progression for divine casters.
Caster level can be increased past character level if the player has levels in prestige classes that grant caster level. A Sorcerer 20/Arcane Scholar of Candlekeep 10 for example will be caster level 30 but with practiced caster he will be caster level 34.
The practiced caster feat works correctly when combined with non-casting levels or levels of other casting classes than the class the feat is targeted on. For example a Fighter 3/Wizard 4 would be a caster level 7 with practiced caster or caster level 4 without. While a Cleric 6/Harper Agent 4/Fighter 1 would be caster level 13 (6 from cleric, 3 HA, 4 PC).