Hi Harman - Sorry you are having to deal with this, but I will hopefully give you some positive views!!
I did both my 1st 2 pregnancies alone, really out of my own choice though - same father I hasten to add, we got back together when ds was 2. I split with him at 9weeks into 1st pregnancy and 5 weeks into 2nd. And believe me it is I think easier sometimes. The most important thing is that you will be able to bring this baby into a stable environment, which is better than if you split later. What will be hard is that you have 3 others, hopefully they will be able to benefit from the situation being resolved for good.
Ex-p had a very good relationship with ds, but in retrospect I think ex was trying to get back with me all along. While I was pregnant with dd he met someone else (which I was fine about, after-all it was my choice to go it alone). And unfortunately he had very little contact with dd, and now no-longer sees ds either. ex-p now has a baby, so this might explain why.
Your situation is different, he has been there with his other kids, so you have a bigger chance to keep him involved. My sister split with ex-p when she was 5 months with 2nd baby. And their father still has reasonable contact with both, and treats tham both equally. He was at births though, even for 2nd one, so maybe that helped. My ex was not at births of either of mine.
Maybe this doesn't sound so positive now I've read it. But if you can keep talking and be amicable at least around the kids, then there is no reason why he should treat his 4th child any differently.
You say he wasn't that involved in the pregnancies, how was he once they were born??
And have you decided on any regular access for your other kids?
I know at the moment you feel like not having any contact, but it is important that your unborn child does not feel that he/she was the only child that the father rejected.
As I have found out, a father with all the best intentions in the world, saying his ds is the most important thing to him, can change with a drop of a hat!
On a lighter note, I am getting married to the love of my life in 5 weeks, and we are expecting twins at the end of this year....
I apologise if this all sounds disjointed, but if there is anything particular you would like to ask then please do.
xxx