Ofw 4.90 is not necessary, instead install
HFW 4.90.1 from OFW 4.89 (HFW is a modified OFW)
Once on HFW 4.90.1 use the new
Flash Writer for 4.90 to jailbreak your console.
Make sure that you make a dump of your flash memory on USB.
Use
PyPS3tools checker to verify your flash before to continue, to verify you have a valid backup.
Then the patch flash to allow CFW installation. Dump your flash again to verify it was patched properly.
Install
Evilnat 4.90 if the flash process was successful. Do not turn off your console if the process fails. Ask for help in the forum.
DO NOT turn off your console while the flash is written. It can brick your console if the process is interrupted.