There kinda right. The rs4-3 isnt really worth modifying, it wont really net you much improvement..especially on a parkinglot. Just get power upgrades (motor , header + pipe , radio upgrades) and stuff that you break perhaps look into upgrades then for that part..Otherwise dont upgrade. Maybe get a few spring sets though for tuning, look up the setup sheets on hpi and see what they are using for the rs4-3.
Then get yourself some nice slick rubber tires. Possibly even try a set of jaco nitro foams (you can get them at rcmart.com for next to nothing and they are actually quite good performers).
Be sure to do one mod at a time so you know if it helped or not. Otherwise if you do a bunch at once you wont know what helped or not.
Id maybe also look into a new chassis (id suggest hpi since its the best but it costs 70 so look for cheaper options like on ebay) and not sure if your top chassis is CF / carbon-fiber if not maybe get that upgrade. Other than that....leave everything alone. Keep it clean , spray wd-40 on it every once in awhile , keep the filter clean , dont let dirt get in the engine ever , when your done for the day , run the car till it stalls , then put the glow-plug back on it , try to restart it and keep going until you cant start it anymore. This will ensure all fuel is out of the car to prevent rusting your motor.
Enjoy the hobby , maybe get another body. Then when your ready to race give that thing a try , then next year maybe get a better pro-car. Mugen mtx-4 might be out then , hpi r40 is ok if you get the hara edition its better , kyosho v-one-rrr is also good.