1/3 the aileron towards the inboard side. Probably won't make a difference though being a 4*. If it's a 40 size, I would just use one, even with a 46 size engine it won't matter much. I used to put mine through hell and back and never had a problem with one. The 60 and 120 size need two servos.

PS-If it is the 60 size-clip a wing bay on each side and change the dihedral to 1 degree-you will get a nice roll rate. You can do that on the 40 size but I would leave the length of the wing the same.