FortiGate 40F vs 40F-3G4G

[Gistron]

I want to buy a 40F but also want 4G failover. I read that I can plug in a 4G USB dongle in the 40F, but there is also a model with integrated modem. This model and the licenses cost around twice as much. 

Do I need another license if I plug in a dongle in the 40F or is there any other difference beside the modem that could explain the the huge price difference in the licensefee? 

That the hardware costs a bit more is understandable. Although a PCIe 4G modem is about $50, so either the 40F-3G4G is really overpriced or there is something else.

[AlexC-FTNT]

Can't comment on the price difference, but finding a compatible 3G modem may be a challenge (to find, or to configure an unsupported one):

https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-to-view-the-3G-4G-supported-modem-list...