mounting engine 0n sig 4*
An upright engine will place the carb above the top of the tank. A side mounted engine would place the carb closer to the centre line of the tank, but I am not sure that it is necessary.
I have used an Irvine .53 and an OS FS-52S on mine, mounted upright. The Irvine works perfectly. The FS does not. From the sound of it, it seems to be too rich when the plane is inverted. In theory, this can be expected since a low tank will be too high when the plane is upside down. I have set the needle valve so that it picks up a FEW rpms when I hold the nose straight up, and I don't want to go leaner than that. But there might be a way to set the needles to avoid this problem. Or I could buy a Perry pump.
If I were going to build my 4* now, I would mount the engine on its side. On the other hand, there are thousands of 4*s out there with the engine upright that work perfectly.