FortiGate 40F vs 40F-3G4G

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.