A:Answer Short Answer : It cannot play a 4K Ultra HD Blu-ray movie disc. The product website shows the drive as supporting BDXL, Blu-ray, DVD, and CD media. NOT UltraHD Blu-ray.
I picked up one of these at a BestBuy a couple weeks ago. In my experience, it could NOT recognize a 4K UltraHD Blu-ray movie --- I used a 4K Blu-ray movie (Oblivion) as my test and the drive just kept trying to spin up over and over and eventually quit. The Pioneer BD Drive Utility said that no disc was detected. I placed the disc in my Xbox One S to make sure it wasn't the disc and i immediately had playback.
The disc is an UltraHD 66GB Dual-Layer disk. The drive supports the following read-only Blu-ray disc formats : BD-ROM, BD-ROM DL. It supports a 6x BD-ROM read speed. According to specs, the drive seems like it could support Ultra HD Blu-ray, but it cannot. This drive is good for burning to high capacity discs and regular Blu-ray playback.
Right now (July 2018) a mobile/slim external Blu-ray drive capable of playing an Ultra HD Blu-ray is hard to find in the United States. You'll find plenty in Japan and some in Europe. There are few internal 5.25" drives for computers available now. Looking forward to Pioneer's BDR-XD07J-UHD when it makes it to the U.S.
Sources:
http://blu-raydisc.com/Assets/Downloadablefile/White_Paper_General_4th_20150817_clean.pdf
https://www.pioneerelectronics.com/PUSA/Computer/Computer+Drives/BDR-XD05S#ReadSupport
http://www.blu-ray.com/movies/Oblivion-4K-Blu-ray/156371/