Probably 3mm but could be 4mm screws. If they look almost identical in diameter to 4-40 they are 3mm; larger then they are 4mm. Homedepot sells the 4mm but not the 3 IIRC.
I have the same fan but got it wihtout the motor for use in the Nitroplanes Predator. I'm putting an 1100KV 36 x 70mm Quanuum in mine and running 10 or 12S. I can't remember the specs I got in it when I ran it up but the amps were of course quite a bit lower and watts were similar.

I'm pretty sure Rainbow is the original British company that sold out the tooling and is not in business any longer. It is the same fan sold as a combination by Himax with one of their motors. An extensive thread wtih lots of information is avialable on RCG here: http://www.rcgroups.com/forums/showt...t=922350&pp=20

If you ever want to get one of the fans only, I bought one here for cheap:
http://www.hobbypartz.com/10eldufanun.html