NBA fans in the US will get their first chance to check out a live game in 4K Ultra HD on Saturday night when the Utah Jazz visit the Denver Nuggets. The broadcast will be available via DirecTV if you have the proper package and equipment (Genie HR54 DVR), as AT&T is working with NBA TV on the broadcast.
In early 2015 (pre-Porzingis), the NBA did a very limited test 4K broadcast of a Knicks game, and in January it tried one on UK and Canadian TV, but this is apparently the first one available widely in the US. Keen-eyed forum posters noticed another Nuggets home game listed as 4K in November, but it's unclear whether that one was actually in 4K. Additional 4K games are planned, although there's no word yet on which ones.