Curious - Why a rack? I have the MC6000MK2 and bought a hard carry case with sliding laptop stand built in. I couldn't imagine using it any other way. I tried the rack mount route and gave it up in short order as being bulky and not ergonomic enough for my purposes:
Current rig:
It seems that on this page there is one comment that says the MC4000 will most certainly not be rack mountable...
[NAMM 2015] Video: Denon DJ MC4000 Controller