We're talking about newcomers here.
If somebody hasn't played the series before, we want them to start with something more simple, and that is Wario Land 4, then moving on to other games, having already learned Wario's main mechanics
So, start with 1, which is like a transition between Mario and Wario.
Then Wario Land 4, which allows players to learn a lot of new mechanics (transformation, puzzles, exploration) without ever feeling overwhelmed.
Wario Land 3 is the most confusing game for thosr who haven't played it, so I woukd recommend saving it for aftef Wario Land 2, which will get players used to the no death mechanic