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. 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).
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 and Harper agent will increase caster level progression for divine casters.