Perhaps these AGW wheels '4847NW 3' 11" 15.6mm 12 spoke Plain' will do.
I've edited this post to reflect a change of view I've had about this conversion.
Looking at the S4 Manual the overall width of P4 wheelsets (back-to-back plus twice the tyre width) should give you 19.75mm max. 'Kier Hardy' shows in his YouTube film that the space between the frames is 21.26mm which should give enough space to fit P4 wheel sets. The weight diagrams for Classes 08/09/10/11 have been published in 'A Pictorial Record Of The Diesel Shunter' (Colin Marsden/OPC), the wheel spacing is the same for all four and both Brassmasters and Bill Bedford do nice coupling rods for them.
Still not convince about actually buying this model, I've got so many Bachmann 08/09 in stock. Annoying.