I have a FX 3 and my Cross bike has Tiagra. There's a marginal difference in the shifting, but it's not huge. The FX 3 is a damn fast bike and I love it. I've never ridden a FX 4, so I might love it more, but I don't know about $400 more.
I don't know what the other poster is talking about--I never see FXs on CL, it's a really rideable bike, and I don't think they sit much.