Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine column of August 10, 1997, Marilyn claims that two males and two females is not the most likely distribution of gender in a litter of four puppies.
Here are the actual numbers:
Assuming that each puppy is equally likely to be male or female, a litter with two males and two females is the single most likely combination, with a probability of 6/16. Three males and a female is less likely, with a probability of only 4/16. Three females and a male also has a probability of only 4/16. So each of the splits you've listed is less likely than the split listed in the original question. Only when combined do they become more likely, and I don't see anything in the original question that suggests such a combination. (Four males has a probability of only 1/16, as does four females.)
Thanks to Charlie Kluepfel <ChasKlu@aol.com>, Noah Morrison <firstname.lastname@example.org>, and Dale Hoyt <email@example.com> for writing to point out Marilyn's error.