if you have over 100 or so psn games i would suggest you stay on 3.55, why? because all those eboots you would have to re-encrypt for 4.21, that is a tedious task, till then i am staying on 3.55 because some older psn games will not have the original eboot anymore and i don't have time for such linearities.
if you like to use ps3usercheats that will stop working too so if you use that frequently i would suggest you hold off that "upgrade" but if you don't care about such matters then go ahead and upgrade your cfw if you wish, i personally don't see any benefit yet until stuff mentioned previously is sorted out but that's a million to one chance of that ever happening.
PS3 Slim v3.55.4 CFW Rebug 1TB Internal, External 2TB
XBOX 1 with XBMC
Wii 3.2E + Cfg UsbLoader External 1TB
DS lite & DSiXL + Cyclo ds 8gb micro sd DStwo 16gb micro gd
PS2 V14 legit matrix infinity modchip
PSP slim 2006 5.00 M33-6 8gb memory stick pro duo
PSPgo with 16GB M2 6.60 PRO B10