Let's divide this problem up into cases:
Case 1 (2 1's, 2 2's, 3 4's) - There are 7!2!2!3!=210 cases.
Case 2 (1 1's, 3 2's, 3 4's) - There are 7!3!3!=140 cases.
Case 3 (1 1's, 2 2's, 4 4's) - There are 7!2!4!=105 cases.
Case 3 (1 1's, 2 2's, 3 4's, 1 3, 5, or 6) - There are 7!2!3!×3=1260 cases.
So, there are 210+140+105+1260=1,715 sequences.