You do not say what operating system you have, so try this.
Download a windows 98 start up disk from click here and put it on a floppy disk.
Disconnect your main hard drive by pulling the ide cable from the back of it.
Start the computer with the floppy disk in. Let the programme load and pick start with cdrom support.
Let it load to a:\ and type format c: after it, so
a:\ format c:
this should format the 4Gb hard disk as long as it's not NTFS.
When done, shut down the computer, connect the main hard drive and restart the computer. Your drive should be now ready.