{1, 2, 3, 4, 5, 6, 7} 

Choose a pair of odd digits. 
That is, choose 2 from {1,3,5,7}. C(4,2) = 6 ways

Choose 2 positions for them to go in the 4-digit number. 
That is, choose 2 from {1st,2nd,3rd,4th}.  C(4,2) = 6 ways

Choose the order to put them in the 4-digit number. That is, put the 
smaller odd digit left of the larger odd digit or the larger odd digit 
left of the smaller odd digit.  That's 2 ways.

Choose a pair of even digits. 
That is, choose 2 from {2,4,6}. C(3,2) = 3 ways

There are only two places left for them to go in the 4 digit number.

Choose the order to put them in the 4-digit number. That is, the 
smaller even digit left of the larger even digit or the larger even digit
left of the smaller even digit.  That's 2 ways.

Answer = 6×6×2×3×2 = 432 ways.

Edwin