Convert MV4 to MP4 - 5.9GB file

I am not all that familiar with the first format but to me it just looks like Apple's version of mp4. Try the free Subler and see if it will re-wrap it as mp4. You could also try VLC, but it takes a bit of learning. The trick would be to use passthrough when selecting codecs for "converting" (which you won't actually be converting, just rewrapping). That is a monster file by the way.

Let's get the acronyms straight.

An M4V is an Apple mpeg4 format used to encode videos in the Apple iTunes Store and likely includes DRM (e.g. Apple's Fairplay) copy protection. Due to the latter, you may not be able to convert the video and it may be restricted to the Mac and account where it was authorized and purchased.

MV4 has to do with certain computer motherboards/chipsets.

Videos contain audio and video elements. They are packaged in a container such as mp4, MV4, MKV, etc. Those elements are coded with various codecs. For example, many are some MP4 codec variant, and have AAC audio which can also be know as mp4 audio. In order to play, the player needs to be able to understand the container and be able to decode the codecs. So let's say you have a video with mp4 video and AAC audio. That could then be packed in various containers, including mp4, MKV, etc. I don't have any M4V videos so I don't know much about them, but it could contain plain mp4 codec components inside a package type your TV doesn't understand. If you can use a tool to re-package the components into something your TV can understand then the contents may not require "converting". Converting = quality loss.

Passthrough just says to keep the audio and video elements unmodified and repackage them into your target container type. If you're lucky then that's all that need be done. Note, however, that if what you are trying to do is play a video purchased from Apple this will very likely not work since those have DRM protection. Nothing will convert those and if you want to watch the video on your TV then you may need to try something such as use the HDMI output to feed the decoded signal to your TV. Even then I am not sure if that will work, but I don't have any videos from Apple so I can't check.