I haven't played enough of it to judge, but my understanding is that it's generally considered the worst of the bunch. But to be fair, I hear a lot of good things about Wario Land 3, and I don't much care for that one.
I own SML3:WL, but the battery is dead so it won't save. I really want that to come to the VC so I can actually play it. (Wario Land 2, too. (Tutu?))
I don't even understand how to play Wario Land 4. I was trying to get into it but the progression is confusing to me. And I got to one stage where you jump down into this pit that I absolutely could not figure out how to get out of. It was a big pit with actual enemies and platforms and stuff in it, so I don't think it was a glitch, but I couldn't for the life of me figure out how to get out of it.
I played about an hour of Wario Land 4 and it was pretty fun, but playing GBA games on the 3DS annoys me, they need to add the game-freeze function and a sleep function... I'm too used to closing the lid whenever the hell I want and having my games still waiting there for me. The fact that GBA games just keep running with the lid closed is eerie.
@Anand I'm really spoiled by playing GBA games on the SP+, they don't feel right on anything else, even a DS Lite. And for some reason, like I mentioned, the fact that the SP+ doesn't go to sleep when you close the lid doesn't bother me, but it really bothers me on the 3DS! Why? Dunno, I'm strange, maybe it just irks me because I know the games are being emulated and it seems like a feature that could be easily patched it. I want GBA games on the 3DS to have the same features as the GB games!
I got Wario Land 4 from the ambassador program, played it, loved it, 100%ed it, and now I'm going to go back and play the whole series. I've beaten Shake It!!, so I'll be 100%ing that one pretty quick.