I think you mean what motor you want to put in the HPI RS4 Rally. Lets get with the car first thing, then racing later
The HPI RS4 Rally..although being around for a long time is quite an awesome on/offroad vehicle. Its quite versatile and can take quite a beating. Although it has belt drive, the car has an inner body that couples with the chassis to keep things clean. Handling is pretty cool and its a light vehicle. I've seen folks getting great air time unlike a nitro rally. HPI claims for fast speeds, use a 17T mod motor with the stock gearing. Try a speedgems2 in there. A brushless motor isnt needed and they are expensive. For an ESC, a Novak Explorer is a good place to start or if you want a better one, a Novak Fusion as it can handle both Stock and Modified motors with ease.
For racing, you must learn how to be 'fast' around the track. The fastest person is the one who drops the lowest lap times, not the one with the fastest powerplant in their ride. Start off with the stock setup, then later on you can ask for setup tips from the advanced or pro's. Hope this helps.