Kaos,
If the school you are waitlisted at is your number one choice, then it cannot hurt to let them know that in a letter that describes your interest. You don't have to promise that you will attend this early in the game. But it can't hurt to restate your interest in their program, and show them your hope in transitioning from the waitlisted to the acceptable pool. You can do all these things as you wait to hear from the four other schools. Odds are, you won't hear about waitlist movement to atleast April, and realistically speaking..you should expect to wait into May, June, and July. That is when waitlists start to move. After May 15th, a person can only hold one acceptance. So those that were holding more then one have to decline some schools offers..then those schools look at their waitlists. Being waitlisted still gives you a VERY good shot at an acceptance, but it means a painful wait.
I'd write update letters to the school that waitlisted you (along with updated transcripts, LORs, etc). Then, if you get accepted to a program that you'd rather attend, you can take your self off the waitlist.