What's the difference between the two? We've got a Chromecast in our living room but would like to add something for Netflix in the bedroom. Recommendations?
4k would be the latest HD standard, and if you don't know what it is, then your TV is probably 1080p, you pretty much have to pay extra for any 4k video on most services, so you would know about that too.
802.11ac is the latest wifi standard for fastest speeds, you would have to have bought a router capable of doing that, which is possible, but you likely don't care about the extra speed.