Just saw this on the Wario subreddit, so I thought I'd ask on the forums too.
Personally I'm from the UK, as anyone who saw my profile knows quite well.
Wario Land 4 is a weird one, since I think it could probably be counted as both the easiest and hardest game depending on what difficulty level you play it at.
Including Wario World and Wario: Master of Disguise?
Personally I'd say Wario Land 4 is probably the easiest one (at least on normal mode), while Wario Land 3 would be probably on the more difficult end.
But what do you think?