How are you trying to play the movie? Are you using an Internet provider, or using the Roku Media Player to watch something from an attached USB device or a media server on your network? And exactly what do you mean by "problems"?
Virtually every 4K video you can buy is going to have lossless audio, which your TV (and no Roku device) cannot play. If you have a 4K movie that you have ripped from the disc (perfectly legal to do with media you purchase) you need to do one of two things: either convert the file to convert the lossless audio to regular Dolby Digital, or stream the movie from a media server on your network. There are a number of free media servers available that can do the job. The media server will convert the lossless audio "on the fly" so you don't have to mess with the file itself. I support Serviio DLNA server (disclaimer: I'm a moderator on that forum), but there's also Plex and Emby (both have their own Roku channels for playback).
Dan
Roku Community Streaming Expert
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