M2TS (MPEG-2 Transport Stream) is a Blu-ray BDAV container format that uses MPEG-2 transport stream and comes with a .m2ts filename extension. It can manipulate audio, video, and other metadata such as disc menu and subtitles. At a bare minimum, any Blu-ray player should play M2TS files easily.
M4R is a filename extension for AAC format, which is developed by Apple Inc. It is typically used to save custom iPhone ringtones from Apple iTunes. M4R can only be created from iTunes purchased songs and is usually DRM protected. It can open on iTunes, VLC Media Player, Apple QuickTime Player, and more.