What is the video compression that is associated with the .mp4 file extension? Is that video compression AVCHD (MPEG4 AVC/H.264)? If it is AVCHD, remember that AVCHD is not supported by Premiere Elements until version 7. There are no back patches for that.
Are you running Premiere Elements 3.0 as the 3.0.2 Update?
And, on what computer operating system is your Premiere Elements running?
If AVCHD and version 3.0/3.0.2 are involved here, then discussion of conversion to another format would be appropriate.
Please go into details of the properties of your source media (those .mp4 files).