Can I still roll with the old clients from 4.2-branch even tho Im upgrading my server to 4.3.4?

Yes, the changes on the client side from 4.2.x -> 4.3.x are mostly just the renaming from Hobbit to Xymon. There have been a couple of minor fixes - memory reporting on Solaris 10+ is one I remember - but for the most part the clients haven't changed, only the analysis part which is on the Xymon server.

Regards, Henrik