It's really easy to upgrade to 4.xx CFW, just follow this guide and you're good to go:
1- Make NOR/NAND dump (optional):
Install latest version of multiMAN, plug in a FAT32 formatted USB flash drive into the right USB port, launch multiMAN under Games and go to mmOS->Select any file->Open in HEX viewer->[SELECT]->[START]->DUMP LV2(NO)->DUMP LV1(NO)->DUMP FLASH(YES)
2- Install Rebug 4.30.2 from XMB:
Download Rebug 4.30.2, verify the md5 hash, rename the PUP to PS3UPDAT.PUP and put it on your FAT32 formatted flash drive in PS3\UPDATE and go to System Update and Update via Storage Media. Make sure there's no original game disk in the drive and that you don't Update via Internet or else you would risk installing OFW higher than 3.55!
3- Install Rebug Update Package 0.1:
Download Rebug_Update_Package_0.1_4.30.x.pkg, verify the md5 hash, install it from Package Manager, launch it under Games, follow the on screen prompts, the screen will freeze for a while when you press Yes, do NOT force shut down your PS3 at this stage or else you will get a nice and shiny RSOD! Just be patient and wait for it to finish what it's doing, it will reboot if needed when finished. Once the PS3 has rebooted you can delete Rebug Update Package from your XMB as it is not needed.
4- Enable QA and spoof to 4.40:
Install Rebug Toolbox from Package Manager, launch it under Games, go to Utilities and enable QA. This is to ensure easier and safer downgrading. Now go to Selector and change System Mode to Rebug, this will spoof to 4.40 to prevent accidental updates and allow PSN access. You can also change XMB Operation Mode to Retail and Debug Menu Type to CEX QA to ensure you have the Retail menus instead of the Debug menus which you probably won't need.
If you had patched games to run on 3.55 CFW, you have to unpatch them by restoring the original unpatched files and/or installing the latest original unpatched game updates.