I just beat Dragon Warrior IV yesterday. The final boss was pretty tough, especially with seven forms. But overall I enjoyed the game. The automated tactics system worried me at first, but I felt like the AI was pretty smart in handling battles.

So now that I've played all four NES titles, I'm comfortable enough to rank them from most enjoyable to least:
Dragon Warrior IV
Dragon Warrior III (This was close, but IV just beat it out.)
Dragon Warrior I
Dragon Warrior II