Were you using a partition manager like Partition Magic? If so, I think there is a rescue program you could try.
Otherwise, there is a program called DiskManager that often comes with the hard drives it self which is a low level formatting program. It WILL toast everything on the disk though. The only problem with this program is that you have to get the version what matches you're HDD brand (the HDD companies license this program from whoever makes it). I've got versions that will work on Seagate, and Quantum drives if you need it.
If your BIOS does it properly, you could also try using the low level format in the BIOS.
Try and work out what programs recognize the disk as a 4G drive and use those programs.
I hate it when this happens.
- gaffa