Bandwidth
A 4K stream needs roughly 25 Mbps sustained. Speed test peaks are irrelevant; stability matters more than the headline number.
The device has to decode it
Most 4K content is delivered in HEVC (H.265). Older boxes and some budget sticks decode it in software, which causes dropped frames. A recent Fire TV Stick 4K, Nvidia Shield or modern Smart TV handles it natively.
HDR is not automatic
HDR10 or Dolby Vision has to be supported end to end: the source, the player and the panel. If any link is missing, you get a 4K resolution picture without the contrast benefit.
Cable beats Wi-Fi
For 4K the difference is visible. If you can run Ethernet to the television, do it once and forget about buffering.