Looking at the Trek site and comparing details of the various FX models, the 7.4 has a different aluminum frame, 9 speed vs 8 speed, superior front/rear derailleur. The 7.3 has the same tires (width and grade) as the 7.4, but no carbon fork. It is also a 9 speed but has a groupset that is a step up from the 7.2, but a step below the 7.4. Is there one available for you to test ride?

Many people here ride the 7.2 and I don't see a lot of complaints. The 7.4 is a top notch bike and will give you many years/thousands of happy riding miles. No 2014 models available? That would probably get you a 7.4 in your price range.