Open one of your MP4s in the free download MediaInfo. In MediaInfo, set View to Text and then copy the report it generates and paste it to this forum. We'll better be able to advise you once we see that that video file is composed of.
And remember, as Bill indicates, new video formats are being created all the time. Many video formats in common usage today (particularly phone footage) wasn't even dreamed of when Premiere Elements was released 8 years ago!