One way to select prop size:

1. Choose pitch
high pitch = high level flight speed
low pitch = good climb rate and acceleration

2. Choose diameter
large diameter = low rpms
small diameter = high rpms

This is the way I do it. Type of airplane and flying style decides pitch. Then I choose a diameter that puts a suitable load on the engine with the pitch I chose.

If you are using a 10x4 and want more speed, you must increase pitch. If you increase pitch to 8, you will have to decrease diameter to 9 or even 8 to allow the engine to rev up to "where it belongs" (typically 12-14k for a 40LA...?)

The 10x4 is a prop that gives you good climb rate, but it is not the best for speed. So I think, as Geistware and MikeL suggested, that a 9x7 or 8x8 is what you are looking for. But theory is theory, and the best way to find out is to try a few different types and see what you like best.

Hope this helps!
Tore S.