Hi lollystix!
I was in the same position as you a couple of years ago - 3 DS with fairly small gaps, a husband that definitely didn't want any more and unexpectedly pregnant.
Firstly, in terms of amount of work, I haven't found 4 much harder than 3 - I was already living in total chaos at that point anyway.
Naturally you will have all the comments about wanting a girl - incredibly irritating, but I just told everyone it was unplanned and didn't really care either way.
Your DH will be absolutely fine - even my DH (I say 'even' as he was very anti) said that he knew he would love the baby when it arrived, and he certainly does! The boys will love the baby too - there seems to be a really special bond between ours.
Can't comment on the work situation, but in terms of help/emigrating, I manage pretty much on my own with no GPs being particularly involved and DH working long hours.
I think you will be thrilled in the end - just try to enjoy the moment. I really tried to savour every moment (and still do) as I know this is definitely my last.
Good luck and wishing you lots of happiness.