Game replays and other recordings of gameplay sessions usually cannot be converted to a standard multimedia file (for example .mp4) with standard video conversion software. Unless your game offers export to mp4 as default function, the only viable alternative is to watch the replay in your game and use Fraps, Camtasia or some other screen recording software to produce the video.
For example the popular Rocket League game produces replays (.replay files) that cannot be converted to MP4 videos in-game. You have to use screen recording software to achieve replay to mp4 conversion.Open REPLAY file Open MP4 file
Video editing tool for Apple Mac computers
A screen capture and video editing software
A screen recording utility