Use a tool like FFmpeg or Shutter Encoder to export your MP4 as a series of images.
By default, Maya often only supports .avi (Windows) or .mov (Mac) for playblasts. How to Get an MP4:
Before working with heavy video files, ensure Autosave is enabled under Windows > Settings/Preferences > Preferences > Files/Projects to prevent data loss if a codec causes a crash.
MP4 files use temporal compression, which makes "scrubbing" (dragging) through the timeline laggy or inaccurate as Maya tries to calculate compressed frames.
If you use an MP4 on an image plane, it may drastically slow down your viewport. If it does, switch the image plane's display mode to None when you aren't actively using it as a reference. codec issues - Forums, Autodesk
If your video won't load, it is likely a missing system-level codec rather than a Maya bug.
In Maya, create an and check the box for Use Image Sequence .