I've been playing this and really loving it. 3 was one of my favorite ds games. Gameplay's pretty much the same as 3, but it's addressed the problems I had with 3. Which mostly amount to better pacing, both on the rpg side and the harvest moon side. You don't get rich quick, which I always hate in RF/HM games. It kind of makes running a farm pointless if you're rich after a month. On the rpg side there's just more content. The rpg side of 3 was incredibly short. Since I haven't beat it yet I don't know how much larger 4 is, but there's clearly more than 3.
On the downside the townspeople aren't quite as good as 3, but it's a overall improvement to a great game.