For Four-Mallets, I recommend Mitchell Peter's "Yellow After the Rain" or "Sea Refractions"
Though slightly overplayed, they are very entertaining, and fun to play.
If you want a challenge for Four-Mallets, I strongly recommend...
Keiko Abe's "Wind in the Bamboo Grove" (VERY difficult, but extremely impressive if you get it) you should check it out on youtube.
Actually, just about every piece by Keiko Abe is extremely challenging, but some can be sleep-inducing, which is why I recommend this energetic piece.
"Two Mexican Dances" by Gordon Stout is very fun to play, and is a real crowd-pleaser. It is mildly difficult , and both dances are lengthy, I would recommend learning one dance only. The 2nd over the 1st.
For two mallets, I don't know many pieces, but "Flight of the Bumblebee" is a great piece to show off your great keyboard technique, and dexterity.
Sorry for the novel, but I hope I have provided some assistance for you.