Since the question does not involve a specific order of offspring, the probability calculation is:
P = (n!/s!t!)p^sq^t
In the equation n is the total number of offspring (4) and n! is 4X3X2X1, s is the number of girls (2), t is the number of boys (2), and s! and t! are both 2X1. In the equation, p is the probability of having a girl (0.5) and q is the probability of having a boy (also 0.5). So
P = [(4X3X2X1)/(2X1)(2X1)](0.5)^2(0.5)^2 = 0.375
The answer is not 0.5, since there are several different possible families of 4 offspring.