I can confirm from intensive observation this evening that both buggies and wheelchairs can be safely accommodated on the P4 in a shared allocated area. There is permitted space for one wheelchair only per bus which leaves room for potentially three single buggies or two larger buggies, or even a side by side double buggy, a good sized single buggy and a wheelchair.
The wheelchair would go where the headrest is. On the P4, the refurbished single deckers have had the fold-up seats removed from that area which would discourage passengers from sitting next to the buggy.
There's always the risk that Selkent put those smaller buses out (same as the 356) where you can squeeze 2 buggies if you're lucky or 1 wheelchair and a small buggy.