what would be a good 4 mallet bach piece for an undergrad audition?

There is alot out there, I suggest you check out the Sonatas and Partitas for Solo Violin, the unaccompanied cello, and even the lute suites.
But do you HAVE TO play Bach? Tell you the truth, it coudl hurt you more than it could help. People are so personal about the way Bach should be interpretted. In your audition, you have to be able to convince the judges about your interpretation, which is tricky with Bach. But if you can do it, then go for it!
The beauty of the piece won't win you the audition. The best tip I can give you is that well, obviously you need to go into the audition knowing how to play the pieces. BUT, they are really concerned with how you are relaxed in a pressured situation and how you interpret the music. So no matter what you play, just be relaxed, breath, play confidently. Good luck!
~Behzad