What version of Premiere Elements and what computer operating system?
Please give more details of what you want to do.
Generalizing, a video length can be decreased or increased in a Premiere Elements project - Time Stretch, trimming in Preview Windows, use of Scissors Tool, dragging in or out of the edge of the clip.......
If you import an .mp4 video, it can be exported as .mp4 with a variety of choices in the Publish+Share/ version earlier than 14, and we need to talk about the ways in the new Export & Share of version 14.
Please consider and supply the details so that we can customize the appropriate reply.
Any questions or need clarification on the above, please do not hesitate to ask.
I've got Premiere 13. When I edit a mp4, the result has a different format - I think it was .prem or something like that. Windows Media Player and other venues don't recognize the format, so the edited video is useless. I use the scissors tool I think. All I want to do to most videos is to trim a few minutes off the end.
When you open Premiere Elements 13/13.1 Video Editor/New, you are opening to a new project. You import your .mp4 (Add Media/Files and Folders/Project Assets) and drag the file to the Expert workspace Timeline. The project should then set the project settings based on its analysis of the properties of the first file drag to the Timeline.
You can use the scissor tool (the tab on the stem of the Timeline Indicator) to cut the video at wherever you have the Timeline Indicator placed and the clip selected. Shortcut instead of that tab = Ctrl+K.
Now this is the part where you may need help.
The only way out of that project file for that trimmed video is export. And, if you want a .mp4 file, then Publish+Share/Computer/AVCHD with Presets = one of the .MP4 choices (the one that has properties that agree with your source video. If you have a 1080p project and 1080p file, then the export would be Publish+Share/Computer/AVCHD with Presets = MP4 H.264 1920 x 1080p30 (frame rate depending on if you working in NTSC or PAL).
If you post details, I will suggest the customization of the project setting and export settings.
Please review and consider. Any questions or need clarification, please do not hesitate to ask.