Do and/or Should SLAs derive their caster level from the class they got the SLA from or class that they have max caster levels in.

For example ... 16/4 Wiz/Cler w/ T4 DD Negative Energy Burst SLA ... is the caster level 4 due to being from cleric tree, or is it (or should it be) 16 since the highest caster level is from Wiz and its a spell like ability rather than an actual spell.

Thanks for your help.