Well first off I know it will hurt but you need to cut it COMPLETELY off from your Ex. No phone calls, emails, or anything if you are truly interested in this other woman. If she doesn't want kids and you do and after 4yrs that opinion hasn't changed then there isn't a reason to give keep hurting yourself. The biggest thing that always tends to turn most girls crazy is when you Break up, keep talking to them, then are seen or tell them about a new girl. Always happens actually to both guys and girls so it seems.
Never date her Best Friend unless you don't value your balls
It may sound harsh but after I broke up with my b/f of 3yrs in all ways he was cut off. Not allowed to my house, work, or any place where I was at. But then again he turned into a stalker the second he found out that I had another love interest. Its just not worth the pain and heartache you are going to feel. You are already torn up enough from the fact you been together a long time and you will always have a place for her in your heart, but you have to remember you need to make yourself happy also.
Good luck man
*crawls back under my rock*
