The OS .25FX is pretty potent and the OS .40LA isn't and I've seen them in a 4*. As has been said, be careful about tail weight. You can use a smaller tank, which will save fuel weight and allow you to put the battery under the tank.

I would mount the engine reasonably far out on the mounts before I installed the radio. I would locate then after everything else is done. You can move the throttle servo up next to the tank. You can move the aileron servo forward. You might check out pull-pull on rudder since it would add less tail weight. I would also consider pull-pull on elevator or use a hard balsa 1/4 square for the elevator pushrod.

I certainly would not use fabric, which Sig says not to anyway. I forget if all the surfaces are built up or if some are solid. I would build up all of them. Also get a light tail wheel.

Finally, I would clip each wing 1 rib. I would do this anyway for better roll performance, but it will also save a little.

I think if you can keep from adding lead up front, you'll can have a nice airplane.