The inner one on the tailgate was ok, as were the two the other side, but the outer ones are the legal ones, as tailgate ones can 'disappear' if travelling with the lid open.
Not sure on the accuracy of that as there are plenty of vehicles with tailgate only mounted lighting, as number plates
Certain vehicles do have additional lighting so that lighting to the rear is still visible when the tailgate is open, such as the Audi A1 (inside the tailgate enclosure), and BMW i3 (below the tailgate on the rear bumper, but these repeater lamps only operate when the tailgate is open / unlatched and are dead the rest of the time), but that’s more for safety when loading / unloading rather than so you can drive about with the tailgate open.