My understanding is the Version 4 has new electronics (hardware) - the original V3 electronics being no longer available...so if you lose/ damage a V3 then the only option is a V4 replacement.
The Synscan versions are to accommodate to new ALt-Az EQ designs etc.
You could check the Skywatcher site and see what's new in each version?
There is no benefit whatsoever in upgrading to V.4. In fact, two features
have been deleted, one being the ambient temperature display, and the
other I can't remember, but I expect another member will know. Also,
the handset shape is different, and less aesthetically pleasing, and doesn't sit properly in the plastic bracket. I've just remembered the other deletion.
You can now only power up the handset through the mount; before you
could power it up independently.
raymo
I have the 16" goto and fully agree with Raymo - there's no discernible programming benefit in upgrading the handset, and the physical V4 handset design has been carried out by Satan himself. It's even more painful than the V3 version to slot back into its holder in the dark - I've ended up using velcro to hold mine.