[Photonboy]

4K over wi-fi:
I tested the 4K Fire TV with my 5GHz network (Archer C7 router). Router in basement and stick on 2nd floor. This stick has very, very good wi-fi. Here's some info:

1) 5GHz network shows as barely connected in 4K Fire TV setup (I forget the word but a better 5GHz network would give a much better signal)
2) 58Mbps, 4K video played fine despite this weak connection
3) 120Mbps, 4K video had buffering issues
4) 4K BluRay I compressed was only 12Mbps (HEVC/H265) but looked about same as original
5) 4K Netflix is probably less than 10Mbps

*So the 4K Fire TV is very capable to play back 4K content as long as the bitrate isn't too high for your network. Anything streamed over the Internet is probably compressed enough it's a non-issue.

Unless you are streaming UNCOMPRESSED, 4K BLURAY or similar you are likely to not have issues if you have 5GHz. If you only have 2.4GHz I can only report that 58Mbps was too much for me but 30Mbps is probably doable (again depending on router, obstructions and distance).