Prestige classes that grant domains – Effective cleric level

For a prestige class that grants domains and increases caster level for a previous class, if it grants an extra domain, what is the effective cleric level for that domain? For example, if you are a sorcerer, and you take four levels of Rainbow Servant, you gain access to the Air domain. What is your effective cleric level when you turn/rebuke Air/Earth creatures? Is it your level in Rainbow Servant only? Or is it your effective Sorcerer caster level? Or is it your total character level?

Edit: this came up when the Rainbow Servant came across a wyrmling Green dragon, and she remembered she had access to the Air domain, so she turned Air creatures. She is Sorcerer 6/Rainbow Servant 4. If the effective cleric level is 4, she can roll to turn the 5 HD Air creature. If the effective cleric level is Sorcerer Caster Level, then it is 8. If it is Sorcerer + Rainbow Servant levels, then it is 10, and in the last one, it is possible that the little dragon goes pop (I said it was 4 = Rainbow Servant level).