I built my 4*40 with one servo for each aileron rather than a centrally mounted servo driving both ailerons. This makes the plane more responsive in roll, is very easy to do, and only has the weight penalty of the extra servo; it also allows you to set up flaperons if your radio allows. You will find a ton of advice here on how to do this if you search for "dual aileron servos", and I'll be glad to help if you email me. The basic idea is to install each servo on a plywood hatch cover that will go in the rib bay in front of the center of the aileron, and bring both servo leads out to the center of the wing, where you will either connect them to the receiver through a y-harness or to two separate channels, depending on your radio's characteristics. As for the hatch itself, it's easy enough to build your own, but there are some ready-made units as well.

If you don't want to modify the kit for dual aileron servos, the procedure for installing the central servo would be: make sure your servo will fit in the mount before assembling it, making provisions for clearing the side where the wires exit the case (you want a small clearance all around rather than a snug fit); CA 1/8" strips of plywood under the mount (i.e. on the opposite side of where the screw heads will go) so that the screws will have something to bite into and so won't work themselves loose from vibration or stress; dry-fit the servo mount on the wing to ensure that the pieces fit and sit well against each other; finally, glue the mount together and to the wing with 30 minute epoxy.

On a separate subject, the 4*40 is notoriously week in the tail area, particularly the area where the vertical fin meets the fuselage. It would be a good idea to reinforce this area with fiberglass prior to covering, and to install the "optional" fillets in the fin/stab joints.

Good luck,

Juan M. Villaveces