This indicates the interface is connected. Your grep for DENIED indicates there are no denials for /dev/ttyS4. How are you trying to access the device? As root? As non-root? If it works as root but not non-root, check the permissions on the device with ls -l /dev/ttyS4 and/or getfacl /dev/ttyS4.
I suspect either you need to be root or your user needs to be added to the ‘dialout’ group. Note that allowing non-root users to use devices is being discussed in: Multiple users and groups in snaps