If a couple has four daughters the % chance of their next child being a daughter is

50% chance

Explanation:

Humans have two sex chromosomes, the X chromosome and the Y chromosome. Males inherit both X and a Y chromosome, while females inherit two X chromosomes. Thus, the sex of the child is determined by the sex chromosome carried by the male gamete (i.e., X or Y chromosome) which fuses with the egg cell (always containing an X chromosome). In consequence, the chance of having a sperm containing an X chromosome (i.e., the chance of having a girl) is an independent event of previous fertilization events (which led to the formation of 4 different female zygotes), thereby the probability will be 50%.