This is my first post to this forum. I have 6 production servers currently running RHEL ES v.3 and need to upgrade to ES v.4. I would like to upgrade vs. a fresh install. What concerns should I have?

not a vast amount of problems, but with a production environment you'd need to very much plan for a worst case scenario here. rhel3 uses a 2.4 kernel whilst rhel4 made the jump to 2.6, so you may well have issues with things around there, especially if your services are bespoke in any way. as long as you have disk images and such then an upgrade is worth it, and is by design meant to work just fine.

I always perform fresh installs. With that said…

If you are running a database i.e. Oracle, DB2, Informix or applications etc I’d perform a fresh install on a separate server and perform testing. If this is the case there are patches that probably require applying, a compatibility RPM or more installed, application specific settings modified. Read the vendor specific release notes if applicable.

If you are just running canned Linux services only you should be smooth sailing. Regardless it is always good practice to utilize a testing and acceptance plan when dealing with production resources.