They do have similar internal specs. Biggest difference is physical keyboard on D4 and none on Maxx as I would guess was obvious. Maxx has way more battery capacity(almost double) and larger screen. Maxx has AMOLED screen, while D4 has LCD and both are Pentile(check out both screens as they are quite different even though speced similarly). Have to remove backplate to get to sim and MicroSD on D4, but on Maxx its under flapper cover on side. That's about it. Maxx is all about the bigger battery and screen and D4 all about having a physical qwerty, but are quite similar otherwise.
Biggest change from you DX other than LTE, faster processors, and obvious aesthetic difference is the move to a permanent battery and the new display types that are pentile.