This is one of the most annoying bugs I run into. I'll try to export photos from iPhoto 4, and I get an error messages saying there's not enough space left -- when there's plenty of space left.
This error message arises from many bugs, but the most annoying is the 4.21 GB boundary bug. When that's the amount of drive space left, iPhoto croaks on export. Some sort of 'divide-by-zero' bug.
If I duplicate a large folder to get my free space below 4GB the error goes away and I can export.
Unfortunately iPhoto 5 is unuseable on a G3 machine and has a reputation for very nasty bugs.