I'm not really sure on what your asking. But this is my reply. It sounds like you want to put the 4.21 on your 3.10. alright! do it. But if your just trying to transfer data to thus ps3 then update it to 3.41 or 3.55 ofw then update to 3.41 or 3.55 cfw!
Then once done install okg for 9.99 spoofer. Then connect the ps3's with internet cable and simply do data transfer from 4.21 rebug to your 9.99 spoofed ps3! now your cfw ps3 will have your data. I hoped i helped. I've been away from hacking community for a bit!!!