Well, I've done it now, it took a while starting this morning, and now look at the time!
I did a complete reinstall, taking the opportunity to move Prepar3d v4 from my nearly full 1TB drive to half full 2TB drive. This also meant that uninstalling first left only those files in the old P3D folder which were additional to a default install, and they can just sit there until I decide whether I still need them... so far I've moved all the left-over aircraft, which gives me all my addon aircraft with no reinstalling and no clashes.
The installer did stop and ask for more disc space, it tends to use the system drive a lot during the install, and my 120MB SSD is quite tight already. Still, a normal clean up -- the biggest culprit is Spotify, which cache's everything I play, and it can eat up 10GB of my SSD in a few days -- did the trick.
I did remember to reboot after the uninstall, but I did have a few issues after reinstalling, until I had rebooted a couple of times. For a while I was thinking that I was in trouble, but it all settled down in the end.
Most of my important addons use add-on.xml, so it was surprisingly easy to get stuff back up and running, I then ran FTX Central to install HD Trees, but nothing else at the moment.
I will post some screenshots later, but really I'm very impressed with this update. I know a lot of folk have said that this is the 'most improved' update for Prepar3d, but I normally just assume that they are just fragile creatures who want so much to be happy with their sim -- but really there is a big, noticeable jump in both performance and appearance.
I'm still awaiting PTA, but even without it, I'm enjoying the new sim. Initially I thought something was wrong when the alps looked funny, but I'd forgotten to activate the VLC mesh, and it's been a long time since I saw default mesh!
In answer to Cowpat's post below, yes photoscenery looks better, off into the distance, I have some screenshots which show this I think. I even trying slewing fast the southern lakes, and it still holds together well, compared to 4.3 where if I slewed from one side of NZCH to the other I'd need to wait for the scenery to catch up.