I don't know if there was a snap ring or not. I'd already done about 12,000 exposures before the cap came off the first time. When the rubber cap came off I caught it, but if there was a snap ring it could have easily fallen to the ground without me knowing it. Even if I knew there was a snap ring, I couldn't possibly have found it in the artificial football turf. As I said above, just putting the rubber back into the hole, worked for another 17,000 cycles.
I just inspected the sub-selector button with an 8x loupe. The mechanism is identical to the vertical multi-selector. I can't see any retaining ring when I move the button through it's normal motions. I suppose a retaining ring could be hidden under a lip in the rubber, but nothing is visible .
I believe that I could easily install the rubber cap and snap ring myself. I certainly don't want to spend $66 shipping my camera to Melville and lose use of it for a couple weeks for such a simple repair. As a minimum, I think Nikon should pay for the shipping if they are unwilling to sell the part to me.
The high school team I photograph starts their post season playoffs tomorrow. If things go well they could be playing until the end of November. I can't even think of giving up my D4 as long as their championship hopes are still alive.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery