Is it wrong/unadvisable to have a 4th child knowing the only way you can afford to go back to work is to leave the child with your retired parents (who have already looked after the first three 3-4 days per week from the end of Mat leave until school?) They also cover the majority of school holidays for you for the existing three. Child 1 would be close to 13 once child 4 is born so a broad range of ages.