are there any major differences betweem the two to even bother downloading the iso's for fc4 and updating.

I did a yum upgrade from FC3 to FC4 the other day, changed about 550 files and took a good while. So, I guess there are a few differences. For example, kde is up to 3.5 in FC4 and 3.4 in FC3.

I did an upgrade with the fc4 dvd and that made some apps change to a non-mp3 version etc.
If you are going to download I guess you have a decent internet connection; I'd definitely recommend you go with Homey's recommendation.

Might as well wait for Fedora Core 5 to come out in a couple of months. I believe Fedora Core tries to release a new version every 6 months although not always on schedule. If I were you I would wait and save the hassle of updating your system, although the task should be quick and simple, I don't think you will benefit much of installing FC4 now unless you need a specific feature.

I'm not sure whether you'll be able to update from 3 to 5 though (skipping 4 altogether) can anyone confirm that?

Well I tried doing a yum update but there were too many compatibility issues. So I just went to FC3. I think accessrichard is right, I'll just wait for FC5 to come out. Then I'll burn the iso's. But thanks for the input anyways

I'm not at my linux box right now, so I can't look up the details, but read up ion yum and you'll find you need to set some special permissive parametres to do a system upgrade (not just update).

I recently went from FC3 to FC4 and noticed that quite a few problems I had been having are now fixed.
-I can hotplug my USB drive more than once without rebooting.
-My USB scanner (Epson Perfection 1260) is now supported and works perfectly with Xsane.
-The /dev/cdrom link to my device (/dev/hdd) is now present at boot time, solving a lot of CD hassles.
-My previously installed Xine works now without having to export some kind of library path thing first.

I seriously doubt if you would have problems going straight to FC5 when it comes out. I was also just going to wait for FC5. Then, it was really cold last weekend and I didn't want to go out so I downloaded FC4. I'm glad I did, but of course it's up to you. (At 300kb/s, it only took a few hours to download the iso images. It took me a while to find a fast mirror, though.)