According to a thread on the pfSense forum, FreeBSD does have an rndis driver with 4G modems and you can use it by
Extract the if_urndis.ko module from the ISO. Copy it to /boot/modules in pfSense and make the file executable.

Add the following line to /boot/loader.conf.local to load it at boot, create that file if you don't have one yet:

Does this sound right? I would try it but can't find this file.
I added that line to /boot/loader.conf and works like a charm. These are the documentation links:

If what you're looking for is the binary object file for this module, you can find it at /boot/kernel/if_urndis.ko

Thanks for pointing that out. I'm sure I looked for it, but couldn't find it before.