So it sounds like you have a lack of connectivity between your 4948 and your TFTP server once the switch has booted from TFTP.
Rather than using TFTP, have you got an HTTP server somewhere on the Internet you can put the image on, and load it from that? Can you ping the Internet ok?
I would be tempted to format the flash first as well (in case it has some corruption), once the switch has booted into IOS (not from ROMMON):
format bootflash:
Then when the switch is running (not from rommon), something like:
copy http://x.x.x.x/cat4500-ipbasek9-mz.....bin bootflash: