The 737-400 classic is basically the same as the NG. Basically the same overhead switches, same centre console, same engine start proceedure, similar QRH checklists and the similar performance ( generally they both climb and descend at 300kts and cruise .745 to .780, the NG goes higher)
Only the glass on the 734 is not as big/good as the NG and the NG carries a few more punters.
So I can see how flying them both at once should be no biggy.
Now as for the Dash 8-300 and 400? From what I've seen they are QUITE a bit different, in performance, cockpit design and engines.