The most obvious difference is that the -400 has a stretched Upper Deck which extends rearwards to just beyond Door 2 and is accessed by a rearward facing straight stairway, which starts at door 2L on the Main Deck.
The -200 and earlier have a much smaller Upper Deck area (which was originally configured as a Lounge) and this is accessed via a spiral stairway by Galley 1 at door 1L.
JD