Bec, I am a mother to 5 kids, we are on a single income of around 56k. My DH didn't want anymore babies after #4 BUT I wasn't finished. He wasn't ready to put a end to his baby making days & well, #5 came along much to my pleasure. DH was a bit funny for a few days BUT we both know when I fell pregnancy & we both know neither of us used any contraception. I even made a comment about it later & he also asked about ovulation etc. So we were both very aware of the possible out come.

Anyway DH came around & was over the moon with the pregnancy.

We struggle as every family does. Regardless of income all families struggle at times.
Something to think about... A 4th child will mean an increase in FTB. The baby bonus money could help get you back on top of bills you are behind on etc. Not that I am suggesting people have a baby just to get on top of bills, BUT you are pregnant already so its money you can fall back on to help your family.
Also, if you did split over this, remind him he will still have a responsibility to support your children. You wont be taking away your 4yr old son or the new baby, as he is their Dad & will still play a vital role in their lives.

Personally I would be telling him that NO you will not be terminating this pregnancy. He will however need to seek help with coming to terms with this new life coming into your family. If Money is an issue then he can look at things he can do to increase the family income. But really we have 5 kids & manage just fine.