This is a question that has raised a debate between my colleagues and I. It seems that there is no clear scientific proof that people can determine the gender of their children.
Scientifically the man determines the gender of the child. Humans carry 46 chromosomes, 22 pairs (44 chromosomes) are autosomal and 1 pair (2) are sex chromosomes. The two sex chromosomes are either XX for females and XY for males. That means only the father can give the Y chromosome hence determine the gender of the baby. Therefore it all seems to happen by chance, that the baby either gets the X chromosome from dad and is a girl (XX) or gets a Y chromosome and is a boy (XY). All the other theories going round about how things like diet, time of conception etc have not been scientifically proven.
The husband and wife in this case should stop trying because 4 children is enough for 1 family and if they get a boy as the 5th born then the 5 girls might bully him too much 🙂