Is it possible to get France VFR for P3Dv3 working in P3Dv4? I purchased the Basse Normandie region for P3Dv3, but installation of this made a mess of my autogen at the time, so I uninstalled it and have not used it since.....

I tried as well, but no luck so far. It seems there are v4 compatible versions on Simmarket, but no upgrade path AFAIK

Should work ok I think. wasn't the v4 compatible scenery just recompiled using the v4 SDK? AFAIK there were no changes to resample.exe between v3/4 so the bgls and sign should be good to go.

Getting the autogen working ok as well is another ball game but isn't scenery compatibility related...

It's the installation that I am concerned about. It messed up my autogen in P3Dv3, and I do not want that to happen again. Would I be OK if I made a backup of the existing Autogen folder, or is it not as simple as that?

no it's not that simple. Simply backing up your autogen folder will mean you have to manually swap files every time you want to fly there, so be Orbx Central automatically overwrites all autogen files with the Orbx ones when it runs.

You have essentially two options:

a) use Arno's autogen configuration merge utility

b) use addon.xml to include extra definitions

Both require a little understanding, but aren't PhD level....

It's a mess as Orbx, FranceVFR and some other developers all use their own autogen definitions which replace whatever you had displaying previously.

Until v4.5, Autogen Configuration Merger was the easiest way to deal with products using their own autogen definitions. Then LM changed how autogen libraries were dealt with.
However, ACM does not recognise add-on.xml scenery entries and when I first updated to 4.5 it stuffed up my autogen.

I would strongly recommend Kevin's recommendation of adding any extra definitions manually.
 

I'm on 4.5 HF2, have all of the France VFR (v4 tbh, i cannot find normandy v3) and had almost no problems. Ok, if you lose autogen in one region (probably not just one), you just reinstall one region and voila, ag in all regions is back. Don't know why and how, but it helps. And i don't know if it messed with my autogen, cause if it works, i don't even care. 

What "mess with my autogen" did it cause?

Here I have all France VFR scenery the xml way. Simply by using the AO and convert then from cfg to xml.

And every VFR addon has the same Autogen folder too.

You only iinstall 1 VFR product and then copy the Autogen folder from the V5 root to all France VFR sceneries.

Add the Autogen folders also in the same xml files .

https://windowlight.co.nz/godzoneWP/tutorial-replacing-autogen-config-merger-tool-in-prepar3d-4-4/