Most people will advise a 10 x 6 for a 40, but I always use an 11 x 7 Graupner on my 40's its quieter and pulls well. I found that on the older OS 40 FP that this prop gave almost as many revs as a 10 x 6 in all the engine tests in magazines and I have continued with an 11 x 7 on my Enya 40 SS and OS 40 FP and Irvine 40.
There should be plenty of ground clearance with a Wot 4.
What club do you fly with?