While I did not go with the FX 4 (because I'm an FX 6 Roadie* now), I did think long and hard about the gear range on the FX 4 when buying a bike last year. You definitely should be able to install a 42 or 44 Chainring, I think I saw some manage to fit a 46? You could also get a 10-42 cassette (or both).

If you're within your 30 day grace period with Trek, I would consider upgrading to the FX 5 or 6 if that's an option for you. The gear range on the FX 4 is perfect for gravel, but you'll find yourself spinning out frequently on the road.