I am running a 999 reservoir on my S2R shock which is not the same but similar. I cut the bracket off the frame with a cut off wheel, then touched up the area with paint after I filed it smooth. For my coil I made a bracket out of Aluminum and located it under the seat. If you search "999 shock on a monster" you will find write ups with pictures on how to fabricate the coil bracket.
Now there are small issues-- I have to run zip straps around my coil wire plugs because the front of the seat wants to push them off the coil, but otherwise thumbs up.
Now you may have an extra issue-- the S4R reservoirs are horizontal and the S4 frame is dimpled to clear the reservoir. This would not stop me but you must do your research on how to dimple a metal tube. If you are pro, it will look factory.
Finally if this is too much work and you decide to follow my lead and get a 999 Showa, Only do it if you are 180lbs or less and do not mind reworking a shock/ paying a professional. The dampening is not deep enough to control the heavier spring required on a Monster. After reworking mine it is pretty good, better than stock but not an Ohlins DU333. But hey I am in it for 1/3 of an Ohlins and it's a street bike