AS you can see I have 4 boys, at the moment I have 3 boys in one room and the baby on his own, once A is older I will put the younger 2 together etc. We do have a 4 bedroom house but the 4th room is needing a lot of work and I won't allow a child to sleep in it til it is done. (plaster very cracked etc).

Negatives, well not so much negatives but there is a lot more washing etc, but again with you eldest being older she would be at an age where she is more independant and can also help out with chores around the house. Like Triilian said I think the medical costs can be the most, somedays I feel like I'm at the Dr's clinic every month, didn't help that last year my 2nd son broke his femur, and whilst he was in the early stages of that recovery my 3rd son perforated his eardrum. So to say lots of Drs appts.
We also have PHI so that helps a bit at least.

I think from the way you are talking then you are really keen for No 4, any little odd moments where I think to myself what was I thinking are very much short lived. I am really loving the dynamics of 4 (even if No 4 is only 4 months).