Image sampler value limit posterization

Hey guys,

I'm trying to take an images brightness (black and white values between 0-1) and map them out to create a mosaic. The problem i'm having is that its giving me thousands of different values (0-1) and I want to create this out of just 5 or 6 shades of grey tiles. The image will end up looking posterized which is what i'm looking for.

I want to divide the list up into 5 domains but i cant seem to get my list to run though. 

Thanks!