I'm running FC4, having transitioned from FC3.   I've been 

impressed with how stable and well supported it is. FC4 is definitely a better choice than FC3 if you are going to be working with the SELinux (Security Enhanced Linux) extensions enabled (generally a good idea for a server)

I attempted the same transition and had to jump back down to FC3 (Fedra Core 3). Turns out that the gcc compiler on FC4 won't compile most things (it is much stricter), which is not good for a production machine. Also, while Drupal may function on PHP5, things like popular shopping carts, web chat systems, web-mail and web forums do *not* run on PHP5 yet (which is bundled in FC4). Even if you back-rev PHP and gcc, fixing all the library dependancies to try and support both FC4 and back-rev'd FC3 apps is a nightmare.

Before you jump over to FC4, be sure to set up a test server and test *everything* you plan run.

Other than that, FC4 is quite good, and I'm looking forward to apps getting updated!

--Katin