I named the Topo Gotaland v4>>Götalandv4.img.
All v3 maps have unique names, no map is called gmapsupp.
No overwriting is done.
If I change file name of Götalandv4.img to Götalandv4.imgX, all v3 maps will reappear.
I have a lot of other mapfiles in a separate slash folder within Garmin folder. They are all X-ed out (.imgX) and shouldn't disturbe operation. At least it has been no problem pre Gota v4 install.