A cryptarithmetic puzzle

A puzzle where a mathematical equation is written using letters instead of numbers. Each letter represents a digit from 0 to 9, and no two letters can represent the same digit.

TOO + TOO + TOO + TOO = GOOD

Solutions
T = 1
O = 6
G = 0
D = 4
T = 4
O = 9
G = 1
D = 6

PROOF

166 + 166 + 166 + 166 = 0664

499 + 499 + 499 + 499 = 1996