After the upgrade, do you want the data on hda, hdb, or both?
The data won't truly be safe unless you can back it up onto some media that is not local to your machine.
Having said that, if you data is in the typical user data partitons and if you pay close attention to the Disk Druid options during your Fedora Core 4 upgrade, you shouldn't have any trouble.
I suppose if you really wanted to be on the safe side (and you have no method of backing up the data on a removable or remote source), you could: (1) Make certain that the data is on both hda and hdb. (2) Remove hdb from your machine. (3) Upgrade to FC 4. (4) Make certain that the data on hda is intact. (5) Reinstall hdb, (6) Install FC 4 -- AGAIN using the same precautions to preserve the data on hda.