Thinking about it is important, not just costs, but actually dealing with a fourth child, and all that entails, the attention they need, the logistics involved. School runs, nursery, childminder.
For me it would be a hell no, but I'm selfish, I like having me time, friends, socialising, a life basically, disposable income, holidays, weekends away, day trips, I wouldn't choose to spend my forties struggling financially, struggling for time, struggling for everything really.
You also have to consider what happens if the child has additional needs. With every pregnancy it's a risk, could you manage a child with additional needs? The answer to this needs to be yes, as I'm sure you know.