Every single time a person has a baby, it has a 50% chance of being a boy or a girl. Having 3 girls previously will not change anything. These events are independent. So the probability of having a baby girl given that the first 3 were girls is 50% or 1/2 or 0.5.
However, prior to having any kids, the probability of having all girls among 4 children is : 1.2 * 1/2 * 1/2 * 1/2 = 1/16...so the result are different.