Now that's an interesting one..I hadn't seen that load formula before but I think I can give a rough answer.
First thing though...it takes 1.3HP to drive an APC 10x4 at 16.5K which would be close to the peak HP figure (OK, I don't know what engine it is but most nowadays peak somewhere around there).

Now for the nitty gritty's...your maths were out a bit but no matter...and we'll assume they're all APC props.
10x4 load = 4000
11x4 load = 5324
10x6 load = 6000
Now for the tricky bit...we know it takes 1.3HP to drive the 10x4 at 16.5K but 1.3 HP will only turn a 10x6 at 15K. Using that load figure for an 11x4 I come up with about 15,360rpm. It'll probably be a bit less than that because the engine most likely would be running below it's peak HP figure.