yes, of course. It's recommended to QA flag first while you're on 3.55.
what to do:
1) Download Rebug Toolbox 2.0 (link in the first post)
2) Install it on your kmeaw 3.55, run it and enable QA flag (there are pictures in the OP), or you can just use the old qa_flag.pkg
3) Set your cursor on Network Settings and press the key combo (all at the same time):
L1 + L2 + L3 (press left stick) + R1 + R2 + dpad_down You should see Edy Viewer, Debug Settings, and Install Package Files if done correctly.
4) Now, dowload 4.21REX firmware, rename it to PS3UPDATE.PUP and update the system from XMB system update menu. but before installing make sure you check the MD5 hash of the pup to see if the file isn't damaged during download or else.