Benandoli, I am in a similar situation to you. Currently 28/29 weeks pg with DC4. Totally unplanned. I will be honest with you, I was utterly horrified and so was DP.
DD, our youngest, is starting full time school in September, I was planning to go full time at Uni, we had got rid of our 7 seater, everything just felt settled and right with 3 DC.
You do both need to discuss the possibility of not going ahead with this pg
For us we both felt like we were the ones responsible for the situation we found ourselves in and agreed to go ahead.
It has been very hard to feel any excitement during this pg so far. I think we've both had to grieve a little for the future we were expecting to happen and DP and I have both had to make a big effort with each other to keep reminding ourselves that this is a positive thing.
Please don't rush into making any decisions about this. DD was unplanned (we DO use contraception but have a spectacular fail rate!) and we wouldn't be without her for the world and felt that this one deserved the same chance as her.
Ultimately though, it is only you who can make this decision and MN is a great place for support whatever you decide.
Sending you big hugs