When I leave the office my S4 doesn't seem to want to connect to mobile data. So, I end up with no connection at all. Turning mobile data off and on does nothing at all, I seem unable to establish a connection unless I restart the phone.
It happens occasionally in reverse also, if I go to work (or home, it is not exclusive to a single network) my phone maintains the mobile connection and wont join the wifi connection automatically (and sometimes I need to enter my password to connect to the wifi.)
Note, my issue sounds the same as this one, though there is no solution there; Nexus One: 3G/WIFI Not Switching