This is a bit of a stretch...the fact that a Warp 4 engine wasn't mentioned doesn't mean it didn't happen between the Warp 2 and Warp 5 engines. Just go with our theory :-)
No, I mean its more that that - there was circumstantial evidence suggesting there wasn't - rather than an absense of evidence.
Like for example, Earth freighters with warp 2 engines are encountered who talk about how Archer's ship will mean they will have to "upgrade their engine to the new warp 5 engine", etc. Maybe you could argue they didn't specifically rule out other starships in Earth Starfleet having warp 4 engines however. Then there is the warp 5 test program episodes - someone is going to have to go over the dialogue, but I think that at least implied warp 5 was a leap from warp 2 - maybe I'm wrong.