Pick 2, A and B, and weigh them against each other.
If A and B are equal, then pick another one C, and weigh A and C. If A and C are equal, then D is the odd one out. If A and C are not equal, then C is the odd one out.
If A and B are not equal, then pick another one C, and weigh A and C. If A and C are not equal, then A is the odd one out. If A and C are equal, then B is the odd one out.
Done, in 2 weighings.