There are many things that you can do away from your instruments.
You could sit in front of your tv and during commercials just bust out on permutations with four mallets. Like, 1324, 1423,4132, etc... You could also practice your music, think about it in your head while your playing in the floor to it, use the right technique and sticking. A great thing that you can practice at home is technique.
www.vicfirth.com They offer some great marimba lessons on four mallets.
Check out some of the Drum corps website for exercises. Indoor sites like Music City Mystique have great four mallet exercises.
Suggestions from Mystique would be:
1324, Black Magic, Spatial Awarness