Since there are 9 men and each man dances with 4 women (presumably this means each man dances exactly 4 times), there are exactly 36 dance pairs.
Let x be the number of women.
Since each woman dances with 3 men (presumably this means each woman dances exactly 3 times), there are exactly 3x dance pairs.
Thus, 3x=36, so x=12
so 12 women attended the party.