I use iDealshare VideoGo to change video aspect ratio, it can change aspect ratio of mp4 file with or without re encoding.

Step by step guide at http://www.idealshare.net/video-conv...ect-ratio.html

To change aspect ratio of mp4 file without re encoding, you need follow the steps below:

Click the "Profile" drop down button to MP4 from "General Video" category.

Then click "Setting..." button to open the "Advanced Settings" Window. Go to the "Advanced" tab, double click MPEG-4 on the right side of Video Codec to select Copy from the drop down list.