Having an ARP4 now, I will be very interested in your new car Dave. At the moment I don't really see the need to change, however the KH car sounds distinctly interesting so I may be persuaded to part with a pound or two.
Paul, Keep the ARP4! The WRP4 sold by KH was in fact based on my old Plus Four with which I had a lot of fun and made some memorable journeys. Much earlier in my Morgan life I also had an ARP4 which offered a much more intense experience (occasionally too much perhaps) and its one of the few cars that I have owned that I think about regularly even now. I reckon you'll miss the ARP4 if you change to any sort of Plus Four.
Ian (now a happy Caterham driver (420S))