I'm tiling a small bathroom. It's a chipboard floor which I've covered with 4mm external ply well screwed down. Do I need to put PVA on the floor first?

PVA is a big "NO". It is not suitable for anything tiling related for many many reasons. Anyone who claims otherwise does not know what they're on about. There are loads of threads on this subject on these forums, so I suggest you do a search

If I'm being honest 4mm ply isn't really thick enough -sorry! You should be looking for a minimum 12mm-18mm on floors. As with previous post do not use PVA.

Use cement based adhesive and follow the bags fitting instructions. They usually recommend priming joints and edges of boards. But this depends on the adhesive used.

The only use for the PVA would have been as a wood glue coat to stick the skimpy 4mm ply to the base along with the screws!

4mm ply is not suitable enough....not just the thickness but not moisture stable either...

You could use 6mm hardi backer cement type boards if your floor is solid enough( little deflection)...this will give a better base to tile too and keep the height down..

Mapei say you can use thier adhesives with hardibacker...

Yes, quite right Dave. On a broader note its interesting how often after the requirement for correct thickness of board is stated and explained in a discussion about a floor, that it all gets lost and the must word comes up regarding not raising higher than 'x' level - as though the strength requirement is an option. Even with builders sometimes.