every AC is almost similar to the one before, but it's always a pretty nice game.
I'm not following too much the development.
AC4 doesn't look similar to AC3 or AC2.
Ships and boats was in the AC3 and fighting system was from AC2. Lol.
But the ships were used in about three missions in AC3.
They play a more important part in AC4 which gives it quite a different feel. Plus of course the very different settings of these three games make them feel unique when compared to others.