I am using a d4 1200 on a daily basis for 4 years now and it works great, I am not flying with it, but moving it around quite a bit and it has never let me down yet. especially in combination with the d1 it is great, only the sync thing is annyoing, Air and PW don't mix.
what i am doing is PW on camera, one PW connected to the air transmitter, so the d4 is triggered by PW directly and the d1s by Air. I have a cold shoe attached to one of the D4's handles, so the Air transmitter is in close reach to the adjustments knobs. works well, but of course you loose the adjustment feature of Air, which I don't care for anyway, as I am rather sending my assistant around the set.
I am using Tenba Car cases for the D4 and 4 heads, quite good, but only if you keep your gear close to you, I would never have anyone handle them who I am not paying.
there s actually only one feature which I am missing with the D4, that's the independantly adjustable modeling light, which is great on the D1