Sadly, the only way you can "go back" to LR3 is if you kept your old catalog around.  LR4 would have converted your LR3 catalog and renamed it.  So as long as you haven't deleted your old catalog file, you can just uninstall 4 and reinstall 3.

If you've imported files, made changes, etc, then you have a bit more of a problem. 

If you've imported files, you can "sync folders" to import the images in to your old catalog.

If you've made changes, write the data out to xmp files (before you uninstall LR4) and when you re-install LR3 it will read in the changes and update your old one, mostly.  Some settings are not kept in the xmp files, so you will lose them.  For example, collections, etc.

Good luck!

Rob,

   The correct question (for me) would be:  When will LR4x stop being the problem?

I like some of the features of LR4.  Nice.  But it's still dog slow even compared to 3.6.  As for LR4.1rc2... well..  believe it or not, it runs faster on my macbook pro than my pimped out mac pro...  that's just sad. 

Nothing suggested yet by the forums has had much of an impact on useability of LR4.

Ah well.  Documented elsewhere in this forum. 

No, it's the same program.  The only thing that's different is the installer has to look for the previous serial number.  The upgrade version does not change anything in your existing Lightroom 3 installation.  This problem of speed has been around for a long long time.  And I seriously doubt it will be fixed in the 4.1 final release.

Hey Dorin,

  If you're talking to me, both machines are 1920 x 1200...

My desktop is dual screen, but running the program with only one screen doesn't make a difference in speed (other than losing the functionality of the second monitor!).

Cheers!