Until both M4, C AMG (Coupé too is coming), RC-F are not tested, one cannot know which one is faster. Look at the GT-R, it is much faster than it appears on the paper. GT86 is much more fun than is appears on measures (0-100, lap time, etc...). LFA for example does not have any advantage over 458 Italia, yet around a track it is as fast is not faster. In fact, there were rumors of more CF in the RC-F, not only roof and hood, but also doors and trunk and even more. There is a rumour of a hard-core RC-F in the likes of CSL and BS AMG.
In any case, the M4 better be very very good, be the competition has caught up. New RS5 will also come, with big promised weight saving and +480 PS 3.0l V6 Biturbo, Infiniti Q60 (Coupé version of Q50) will also have a sport version, the Q50 Eau Rouge make 500 PS, but the engine is unknown. Then the ATS-V Coupé, and an Alfa Romeo Giulia Coupé (should be called 6C) with Ferrari/Maserati V6. Use of CF is also likely, seen the full CF chassis 4C. Exciting times ahead. M3/M4 has never had that strong competition. Not to talk if Porsche decides to have a similar model.