I have three Ultra models, the 4640, 4670 and the 4800. In my opinion, the 4800 is noticeably faster to use, and also has the Dolby encoder to convert DD+ to DD if your equipment doesn't support DD+. It does have a USB port for playing media. I noticed that when it downloads an update it loads almost twice as fast as my other Ultra models.
Note that MKV is a container, not a type of media. Yes, a Roku can play a MKV file, but only if the codecs it contains are supported. For any Ultra, that would be MPEG2 (H.262), AVC (H.264), HEVC (H.265) or MP4 video, and the audio has to be PCM, AAC (stereo only) or Dolby Digital. If your Roku is connected to an AVR that supports DTS, then that audio will bitstream as well. It cannot bitstream any lossless version of Dolby or DTS audio. I play media all the time, but it streams from my DLNA server, not from a local connected drive. That way I can transcode any unsupported video or audio. It also has the benefit of the media being available to any device on my network. A media drive connected to the Roku will only allow playing the media on that particular Roku, and nothing else can access it.
Dan
Roku Community Streaming Expert
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