Which part gave you that error? Upgrading the firmware manually or rooting?
Hopefully you were able to pull that off successfully. The phone has built in security that does not like downgrades and some people have lost their phones in the process. Once this security feature is tripped, it locks your phone into an unsalvageable brick.
Samsung Galaxy S® 4 MJA Rooted with CF Auto-Root Tutorial
HTC EVO Design 4G™ v4.0.3 Bootloader Unlocked & Rooted with HTC SuperTool v3
If it ain't broke, use a sledge hammer.