.15SS engine in a Nitro RS4 3 RTR.
HPI .15SS, Pull Start, Rotary Carb, should be a direct bolt in for the .15FE. Just use the engine mounts and flywheel from your 15FE.
If you buy a NON-Pull Start Engine, you will need a Starter Box to start the car. Other than that, everything will interchange with your 15FE.
BTW, FANTOM Racing has a Pull Start .15 engine that will fit, is very fast and usually less expensive than the HPI engines. They are both made by the same company.