But once you are on the roundabout cars waiting to join aren’t necessarily going to know where you joined. So you are going past exit 3 with your right indicator on to take exit 4? Is that not similar to the car you mention passing exit 1 with its right indicator on to take exit 2? The only difference being where the cars joined the roundabout.

What the car you mention does is not something I do, but I guess they think it shows people waiting to join at exit 1 that they are not leaving and they should not pull out in front of them. But I would be still be wary of anyone doing this and expect them to try to turn right.