You must have the same version of VE Pro on both computers, not just V4 or V5 but the exact version or it will throw up a mismatch version error.
You can have V4 and V5 installed, although they cannot run together, as long as you use V4 on the master and V4 on the slave, your old projects will still work. But you need to be careful when you install V5, as it will remove V4 (the installation - not the license on the key), so you may have to hide the V4 files when you install V5. I had this by accident, as I always rename my Vienna Ensemble Pro.dll (VST plug in) to VE Pro.dll, otherwise Cubase shows all instances with the same name (as it is too long), so when I updated to V5, I still had V4 there (it's not quite so easy on the slave though).
Also be aware that V4 probably will not open V5 viframes. V5 works flawlessly for me, so I doubt if you'll need to worry about going back to V4.