What exactly do you expect from a 3.55+CFW? You want just the OFW features? If not, then you might as well stay on 3.55, It's safer (latest 3.55+ CFWs have been quite stable though, seems like they aren't giving bricks anymore) and has better homebrew compatibility, you won't be missing anything, you can run all the games released so far (EBOOT fixes) and still get on PSN by the use of tools, such as PSN/SEN Enabler.
I always recomend Rebug CFWs, they have more features than the others and they're a very trustworthy team. Since both 3.55.3 and 4.21.1 versions are pretty much the same, I think you should just upgrade to 3.55.3. In case you insist on going to 4.21.1, you will want to install QA toggle before, to make the downgrading process easier in case you feel like doing It later.
REBUG CFWs: http://www.ps4news.com/ps3-cfw-mfw/r...owngrader-out/
QA Toggle: http://www.ps4news.com/ps3-cfw-mfw/t...e-is-released/
Both CFW versions seems to be safe to install directly from XMB.
To dump NOR: mmOS->Select any file->Open in HEX viewer->[SELECT]->[START]->DUMP LV2(NO)->DUMP LV1(NO)->DUMP FLASH(YES)