1/(2x2x2x2) for the odds of all roosters, 1/(2x2x2) for 3 roosters or hens, etc. It's been quite a while since I took a course that covered statistics/probability material, but I think it's right. 1/16 of a chance for all roosters or hens.
1/16 - 6.25 - 1 rooster
2/16 - 12.5% - 2 roosters
3/16 - 18.75% - 3 roosters
4/16 - 25% - 4 roosters