While it could be system related (would need the full system specs there), after you check that you have the latest video drivers, Rt-click on your Program Monitor. Choose Quality = Highest and Magnification = 100%. Does that help the preview playback?
Good luck, and if not, please give full system specs,
Make sure your MP4 is AVCHD from an AVCHD camera. (Not all MP4s will play properly).
Make sure you create a sequence to match the properties of the AVCHD footage form your AVCHD camera.
If the first doesn't apply, then a consumer editor may provide better results.