Quick update: my new holder (Brodit part no. 511165) arrived today after being out of stock for a few days.
I can confirm it uses the same style of back plate, so I just unscrewed my old holder (for my 3GS) and screwed the new one into the same location on the pro-clip I already had installed.
The fit is pretty good. I use a clear Incipio Feather case for my iPhone 4S and when in the holder, you can still wobble it a bit backwards and forwards. Don't get me wrong, it's not loose and flopping around and there's no risk of it falling out, but I didn't like the wobble - I'm picky about that sort of thing.
So I cut two small strips of duck tape and stuck them inside the recessed bits of the holder, where the phone slots into. It's only a tiny amount of extra thickness, but because of the rubberised and textured finish of the duck tape, the phone is now held snugly and doesn't wobble at all. A tiny and reversible mod which seems to have perfected this holder for my specific combination of case and phone...
