Well both Fedora Core 3 and 4 are old by now. May I ask why the other versions, like Fedora 8 for example, aren't an option?
In any case a clean install is a wise thing to do. A few reasons:
- Fedora, like many others, doesn't actually "officially" support upgrading the system via yum
- Fedora discs offer an upgrade option if I'm not completely mistaken, so you could download the disc set, burn it, boot it and select the upgrade option; this would save you from potential network errors that could interrupt the update. Still a "non-clean install" is not as good as doing the real thing: old files may be left there, the upgrade can break anyway, and it's a known fact that upgrading that way (especially with the package manager with internet reposities) does cause trouble, sooner or later (I mean "later" as in "you notice the trouble later").
- A clean install takes no more time than an upgrade, maybe even less if you've got the discs ready
- You should have backups of your *important* data anyway (if you don't, then you don't consider your data important, so it's all the same if you lose it anyway), so doing a clean install is no problem
- By doing a clean install you'll get rid of possible problems in the system caused by a bad configuration or such
- Getting your old data back is no problem if you have done backups as you should, and getting all the rest back (software, mainly) is a matter of few clicks in your package manager these days
- If you had formatted your partition so that you had a separate /home partition, you wouldn't necessarily have to move your precious data away from the disk during the upgrade; backups are a sane idea always, but with a separate /home you can just leave it unformatted, and instead format the root partition (and others if needed), do a clean install selecting /home to be mounted on the partition where the home directories are, and after the job is done have your data sit there like it used to be, along with all your personal settings, browser plugins and such
The list is probably endless. There are some reasons to do an upgrade trough Yum (or using an install disc set's upgrade option), but for every such reason there are at least two or three reasons to do the job the right way. So if you haven't, back up your data (especially if you don't have a separate /home partition), do a clean install, maybe change your partition layout to have a separate /home so you have an easy, swift and clean upgrade or distribution switching ahead next time, and go for it.
I doubt if you have many upgrades available for Fedora Core 3 or 4 anymore as they're so old, so you really should consider the later versions, the Fedora series (no more "Core" in the name). If you doubt if your machine is powerful enough to smoothly run them, don't worry; they shouldn't need any more machine power than FC3/4 do, they should have a better hardware support, and it is even possible that they run smoother - because the software (like desktops, KDE/Gnome) has evolved too, and hopefully optimized a little. At least KDE3.xx and Gnome2.xx have become faster than their previous versions were, so even though they're heavy and slow, they're not that heavy and slow as some versions used to be. Some of the early Gnome2.x and KDE3.x versions were really a pain compared to what they are today..