Just bought a slim with 3.56

thanks guys... ill wait for 4.21 cfw.. hopefully it comes sooner then later

there is a werid 3.56 custom firmware, but i believe it isnt that good....

google around

anything above that your SOL

Look at this table and see if you are lucky about downgrading.

SKU Models - PS3 Development Wiki

You may not be able to downgrade. But E3 Flasher users in this forum may inform you of this situation.