Lots more information needed to answer your questions and resolve your issue.
What version of Premiere Elements and on what computer operating system is it running?
Do you have the latest version of QuickTime installed on your computer along with Premiere Elements?
Is your video card/graphics card driver version up to date according to the web site of the manufacturer of the card?
What is the brand/model/settings for the camera that recorded the .mp4 videos? What is the project preset that you (manually) or the project (automatically) has set as the project preset to match the properties of the source media? If unsure, please tell us the readings for Editing Mode, Timebase, Frame Size, and Pixel Aspect Ratio (even if they look grayed out) under Edit Menu/Project Settings/General.
can't edit mp4 videos, because they are added as audio files
Are you getting any error message associated with this situation....import generic error, missing video codec, file format not supported, etc?
Please download and install the free codec utility named GSpot and let us know the readout for Video Name, Codec, and Status as well as Audio Name and Status.
GSpot Codec Information Appliance Windows Only.
What is the file name of the video audio .mp4 that you import? What is the file name under the "audio only" thumbnail in the project's media area?
Let us start here and then decide what next based on the details of your reply.
Any questions or need clarification on any of the above, please do not hesitate to ask.