Converting one of the many existing .replay file types directly to MPEG-4 Video format (.replay to .mp4 conversion) will, most likely, not be possible using common video conversion utilities. These programs, typically, are not capable of converting game replays, or game session recordings to multimedia formats, such as .mp4.
Unless the particular game comes equipped with a default function allowing players to export their sessions directly to .mp4 format, you might have to use a screen recording utility, such as Camtasia or Fraps, to record your game session loaded from the .replay file.
Open REPLAY file Open MP4 file