sturedman, When ever you change firmware there is always the chance that your player may stop being recognised and will no longer upgrade. Your only solution then is to short the memory chip to reset your player which will set it back to an ADFU Device. To give you some idea of what this entails check out the Dead Player Recovery Guide
HERE
You should keep to 3.5.37 firmware of the right screen size.
The other problem you may have is that your 4GB player is quite likely a fake and you will find that your capacity has reduced to pehaps 1 or 2GB when you recover.