Virtual Boy Wario Land was just as dark as WL4. In fact, WL4 takes most of its story and ideas from VBWL! Why do people here pretend it doesn't exist?
Wario Land 3 was sensational, but the baddies were a step down from the previous three games. Who else thought most of 'em looked less inspired, and a bit too cute for Wario standards? That is to say, some of 'em looked more like Kirby rejects than Wario enemies... Like the Para-Gooms, ice bears, silk worms, and those really cute fluffy guys that sit on your head/block your vision in The Vast Plain.
Even the zombies looked worse. In WL2 they were slow, slimy, shambling freaks. Really creepy design. In WL3, they're all blocky and simplistic, and slide across the ground awkwardly at a fast pace. Still, there were some goodies... like the freaky robotic mouse that chases you in the sewers (credit to
@ShyGuyXXL below).