Scinario: If the mobile view (default) for each site/sub site is not activated, it gives 404 error when the site is accessed in mobile view (http://Site URL/m).
Solution: Activate MobilityRedirect feature (it is not activated by default) through the command line as it is hidden.
stsadm -o activatefeature -name MobilityRedirect -url http://Site URL/
To test if it is activated browse to http://Site URL/m/
If it is working it will redirect you to a page of mobile view, otherwise it will gives a 404 error.