Why do you want a different version of Poser, if Poser 5 is working for you? Poser 5 and above allow multiple runtimes, so there should be no problem using your Poser 4 runtime in Poser 5, or Poser 12, even.

IMO, the interface of Poser really hasn't changed much since Poser 5. I think you'd find even the Poser 12 interface very familiar.