hypernut 4in1 foil board 140litres 90kg rider. What sail size to use when it's blowing 8to10knots?
total newbie to foiling but windsurfing for years.

Depends on whether you throw full body pumps, or just wiggle the sail and call that pumping.

6.5 to 7.5 depending on pumping effort.

7.5 it is then, thanks

What foil you using?
I just go bigger front wing in light winds if you have that option and keep sails in 5.3-6.0 range for ease of pumping. The sup style boards with forward mounted foils lift earlier onto the foil IMO. Have fun and enjoy.

Agree with changing the front wing to bigger if possible to keep smaller size sail.
Big sails don't always equal easier foiling. They are heavy and produce a lot of drag, which in light winds can almost kill any benefit over a smaller sail.

Ive got a 6.4 and a 5.7 foil specific sails and I actually feel that I can get up and going quicker on the 5.7 sometimes. The 6.4 is better for fluky winds as it seems to keep me going through lulls slightly better but the 5.7 is so easy to move around and pump I can get going in about the same wind.

As the others have said, depends on your foil! I think 7.5 is very realistic in the beginning with a reasonably sized wing (in your case a fast sup/surf wing? SB Supercruiser?) in a bit you can go smaller. I have 4.6 as biggest now from 10kts up in freeride with a 900 wing and 88kgs. (And a 5.7 lying around for when its less, but i dont like such big sails for freeride, I'd rather go racing.)

experiment. Try a full size smaller than what you'd normally go out in and then adjust from there. Here at 6000ft, I need the full size sail due to lack of air density.