My wife wants a 4th baby, but I don't.

We have been talking about it for the last few days (talking strongly). 

I have raised points such as:

 - We will get less time to spend with our current kids.
 - The age gap between the first and the last will be too wide.
 - It will cost more money.
 - She will be out of work longer.
 - We will need a bigger house. And we don't even have our own house yet (we are still renting).
 - We will have to start again. Sleepless nights, nappies, feeding, new cot, car seat, etc etc.

She says:

 - She feels like she is missing something.
 - She has always wanted a large family.
 - Things will be the same regardless of having 3, or 4 kids.
 - She agrees that money will be tighter, but wants to push through it.

Our current kids are 9 months, 2 years, and 4 years. We are young married couple (25), and I have a fairly decent job.

What would be the pros and cons of having another child? 

How can we rationally talk to the other about having/or not having another child?

And what should we do if we cannot agree?


(sorry I don't really know what to tag this).