Using a Firestick with IPTV becomes much simpler when you treat the Firestick as the device, the player as the interface and the subscription as the content source. Each part has a different job.
What to check first
For 4K viewing, the stream source, broadband speed, TV, HDMI port and Firestick model all matter. A 4K-capable device cannot improve a low-quality or unstable source by itself.
Practical Firestick advice
Remote control comfort is also important. Choose a player that makes categories, favourites and EPG navigation easy from the sofa.
Use IPTV apps and playlists only with sources you are authorised to access. A player app does not provide channels by itself.