I'm looking at the F-X5B as well. If you aren't storing the body and have one lens mounted at all times, the other 3 lenses should fit fine. You can probably squeeze in 1 or 2 pancake lenses ontop of that too.Well, I don't plan on storing my body in the bag, but I ran the math, and it seems like it would be a tight fit.
Total width: 10.9in, a good inch a half over the width of the bag. That 100-300 is the deal breaker, I think. It's a shame, because it seems like a great bag, plus being able to belt mount it is a good option for me. Hmm... I guess one of those lenses would probably be on the camera so the space would be there, but I might have to just wait until I actually get my 100-300 before purchasing.