This is only changing the extension. The videos are already being recorded in an mpeg-4 stream, they are just being created with a .3gp extension. This is really only for people who need the extension to be mp4 for some specific reason.
I have tested this a bit so be sure to back up your original HTCCamera.apk just in case.
Download it here.
As you can see below, nothing with the encoding has changed.
mv /system/app/HTCCamera.apk /system/app/HTCCamera.apk_
adb push HTCCamera.apk /system/app/
chmod 0644 /system/app/HTCCamera.apk