Converting .yrp files to .mp4 format.
It appears that converting YGOPro Replay Files directly to MPEG-4 Videos (.yrp to .mp4 conversion) is not possible, since YRP files do not contain any multimedia data. They are essentially text files containing instructions logs for the YGOPro software.
The only, less elegant, option is to play back the YRP replay file and use a screen recording tool, such as Fraps or Camtasia, in order to generate an MP4 video.
In a word, direct yrp to mp4 conversion is not possible.
So don't expect any so-called yrp to mp4 converter or yrp to mp4 free online conversion tool available.
A screen capture and video editing software
A free online dueling system made for playing Yu-Gi-Oh! duels