There are two choices of which way the men and women sit: MWMWMWMW and WMWMWMWM.
Within those two choices there are:
1. No. of permutations of 4 men;
2. No. of permutations of 4 women.
This is one of the cases where you multiply them all together, so there's 2 x n x n where n is the answer to 1 and 2 above.