I am not aware of a single one. Given the few mainstream releases that are available on UHD Blu-ray and the extremely few adult titles on Blu-ray, I wouldn't expect any in the near future.

Maybe hoping for full on 4k HDR XXX is a pipe dream. Porn companies would have to pay Dolby and DTS licensing fees. And would need to have guys versed in mastering Dolby Atmos and DTS-X. Will XXX companies pay the fees? Not likely. Too expensive. So all we'll get is 4k res online video clips. Easy to do and cheap. Just need a 4k camcorder.
I just naively thought since porn companies lead the way back in the VHS days and I think DVD days...probably more cost effective back then too..

It will never happen. I already covered this in the Blu-Ray thread, so I won't bother posting it all again. To give you a short answer, the adult industry does not and will likely never have access to UHD-BD replication due to the collapse of the optical disc replication industry and a variety of economic factors that make it a bad business choice. The industry can offer 4K HDR downloads in the next four years, but it depends heavily on consumers having updated hardware and content makers investing in the appropriate equipment to make it happen. As of right now, all the cameras used to shoot 4K content support some form of HDR, which can be dealt with by current software, so they just have to buy HDR monitors for grading and testing, which will be out this year. Also, they are waiting on a new video codec that can do 4K and HDR, while also costing no money to use. HEVC was locked out because a group of companies threatened to go after the adult entertainment industry and other content providers if they used it. Thus far, nothing has happened in regards to that, but it's not something the industry is willing to risk and only the most recent computers support it, so most people can't even use it yet. It's always a chicken/egg situation.