Since it's pretty much the standard resolution, 4k viewing isn't hard to come by in streaming. Many on-demand services include it among its features, but live TV is a trickier thing. Live feeds require more data and faster speeds, plus they're often more expensive than on-demand services, so getting 4k as an add-on might be heavy on the wallet. Still, once you watch live events—especially live sports—in 4k, it's hard to go back to anything less. In this case, the best live TV services streaming in 4k UHD are YouTube TV, Fubo, and DirecTV Stream.
For YouTube TV, you'll have to purchase the 4k Plus add-on, but if you can spare paying an extra $10/month, then the service allows you to watch select channels, including ESPN, Fox Sports, Nat Geo, and FX, in crisp and clear 4k. The add-on also gives you access to unlimited streams and offline DVR viewing. Meanwhile, Fubo and DirecTV Stream are perfect for viewing even more sporting events, although you should note that they only offer select events in this resolution. Hulu with Live TV and Peacock Premium also have 4k, but as of this writing, they're sadly limited to their on-demand library.