I’ve been searching in the documentation and i can’t find the name of the interfaces, just know that the /dev/ttyUSB3 is AT command port.
It depends on enumeration scheme. For DIP mode with (default) USB composition #3 selected, /dev/ttyUSBn ports are expected to be mapped as follows:
n=0 - HIP (CnS);
n=1 - QCDM (diag);
n=2 - NMEA;
n=3 - AT command;
n=4 - modem.
As an additional info, please refer to USB developer guide for detailed explanation of the interface available.
Please note that there isn’t necessarily a 1-1 relationship between the MCxxxx serial interface numbers and the Linux /dev/ttyUSBn devices. The latter number space is shared between all USB serial devices in the system, and there is no guarantee that the AT port always will be /dev/ttyUSB2 (or whatever).
Sierra Wireless use static USB interface numbers to indicate different serial functions. The correct way to map these to static device names is using udev to create symlinks based on USB device and interface number matching.