Unfortunately, you've posted to the Tips & Tricks sub-forum, where your post is not likely to be seen by most forum subscribers.
First, (download and installation problems aside) your computer could well be too underpowered to effectively run PrE 8, and also edit MP4 material. Please list the complete specs of your computer.
Next, please list the full specs. of your chosen Project Preset and also the specs. of your MP4 footage.
In the meantime, this troubleshooting ARTICLE might be useful. It's written for PrPro, but there are strong similarities in the two programs.