The fourth child is not expensive at all in the beginning. You will already have all the baby stuff you need, but as the years go by the costs get very steep.
One of the biggest expenses of having a fourth child is getting a big enough car to hold a family of six.
The cost of food rises as they get older. Once they reach about 12 they start eating adult portions, so you have to get creative in your cooking and shopping.
Unless you can afford help, the kids have to keep their rooms tidy and learn to pick up after themselves.
You will end up doing laundry every day. A decent washing machine is essential.
When the kids are young there is a lot of passing clothes on from child to child but as they get older this changes.
Most Irish primary-school books get written in so they can't be passed on. This means the cost of school books rises each year. In secondary school the price of books in 1st year and in 5th year is horrendous, but the books last for a few years. If the children are less than three years apart you will need two sets of the same books.
If you have two or three children doing the same activities there is usually a break in the cost. It is really worth your while to find activities that the children can cycle to themselves or you will end up being a taxi service.
You will never get another package holiday as they are mostly designed for two adults and two kids. It is also extremely difficult to book hotel rooms. A large tent is a very good investment.