MP4 file - change default frame width in file?

[CompVid]

I have an MP4 file which should display as 4:3 but instead default to 'widescreen', therefore everything is stretched horizontally.

I can easily change it to 4:3 by pressing 'A' when playing it using VLC, however is there a way to edit the MP4 to make it default to 4:3 ?

I see from looking at the file properties that the Frame Width under the Details tab is set to 720 so it's that which I need to change?

[ProWo]

Use clever Ffmpeg-GUI, small, portable, no installation.
No recode will be done.
Load your mp4, click main, click change DAR, set value to 1.33 (=4:3), click change DAR.
Done.
You will find your new 4:3 mp4 into the target folder.

[CompVid]

Excellent, many thanks.