The largest possible even number will end in an even number, and have the largest possible first digit. 5 is the largest digit, so make that the first digit. Since both 4 and 2 are even, so either could be the final digit, use the 4 for the second digit, because 4 is the largest remaining digit. The number has to end with the 2.
5412
900 greater than that is 5412 + 900 = 6312