(Sort of a typical Top 5 Highlights list, but whatever)
Probably my best WL4 soundfont work arrangement-wise would be this incredibly obvious Asian spoof of Wario Land 4's Domino Row theme:
The one that I want to hear being applied into an actual WL4 fangame the most vehemently, for obvious reasons, is this remake of Super Metroid's Brinstar themes (with the Upper portion simply being a rather impressively spot-on "80s rock" take on the original theme while the Lower portion is just plain otherworldly spectacular and easily, by far, one of the best takes on both the song itself and its original NES Kraid predecessor to date):
The one that is the most underrated by a landslide (including by its own creator as well) would have to be this Courage TCD medley:
And the one that nails the overall tone of the original song(s) that it's cloned from the most would have to be this Techno Boss Battle remix:
Followed closely by this medley of Geothermal, Labyrinth Fight and Moonsong from Cave Story:
(I'm posting these all at once mainly because the levels/bosses that they imply the potential future inclusion of would mostly all be fantastic ideas for a new Wario Land fangame, barring Nowhere due to the fact that it would make absolutely no sense as a Wario Land level; the songs themselves are also especially impressive for being made in LMMS, and if remade into better sound QUALITY, every single one of them would be borderline perfect)
Oh, and just as a nice little bonus, here's even more proof of just HOW great of a song remake Lower Brinstwar really is: