According to Microsoft support website, Windows Media Player still won't support MP4 files. Though we can make WMP play MP4 video by install some codec, sometime we still encounter can't play MP4 in Windows Media Player issue. MP4 is a container format which may contain multiple video and audio streams. And the video and audio stream can be encoded any of several ways. If the MP4 video or audio codec is not WMP installed codec supported one, Windows Media Player won't play MP4 video of this kind.
In order to solve "Windows Media Player won't play MP4 files even when the appropriate codec is installed" and to successfully play any MP4 video in Windows Media Player, we had better convert MP4 to Windows Media Player more supported video format like WMV, AVI.