I can't agree with this; a) late 3e -> 4e seems like a larger jump than late 2e -> 3e in that 3e for better or for worse preserved a lot of the look and feel of the old classes, spells, monsters, etc., and b) I don't agree that core 2e is that big of a difference from 1e, even core – you can (and I seem to remember doing in a few really suboptimal situations, but it's been a long time) play 2e PHB with 1e Monster Manual, and I know I played 1e PHB with Basic/Expert modules, and things made enough sense to work without much rewriting. Ogres and orcs stayed ogres and orcs. Now, obviously crossing the 2e PHB with the 3e Monster Manual is going to result in a big ol' mess. Whatever the virtues of 4e, it's in a totally different language from 3e, and you aren't going to be able to play for example The Sunless Citadel at all with 4e unless it's ported. There's a metric, anyway. I don't think comparing core editions is very useful.