To go from four to five kids?

No I wouldn’t. I understand you wanting your DD to have a sibling and because you don’t feel done, but really you should have thought about that before having a child with a man who already has 3 children.

They may not get on. You can't guarantee they're going to be lifelong friends.

So, one way or another, I can totally see the rationale for having another (I'd want to if I were you) but it really depends how anti the idea your DH is. Life is so hectic these days and even if he goes along with it but his heart isn't in it, having a reluctant co-parent could make things pretty miserable for you all

I come at this from a different perspective (I'm DC 5 in a blended family) so I wouldn't exist if my parents had decided not to go for a fifth. I have really good relationships with my older siblings (who had a different mum) and my next sibling up (same mum) is very glad to have had an additional sibling. But, the big difference here is that my DF was very much onboard with the whole thing - he was a very present dad, in fact probably better than he had been with my older siblings. But the age gaps were bigger.

No. DP doesn't want to so it's a simple no. Siblings don't always get on either or bother with each other at all.

Her partner doesn’t even want a fifth! Theres no going for it about it!

I would not have been with someone who had 3 kids already if I wanted 2 kids. Clear as day that someone with 3 would more than likely be done with 4.

A friend was in a vaguely similar scenario, but he only had 1 previous child and they had 2 together. Things you may not have considered:

-No history of twins, not IVF pregnancy, but the chances grows as we age. They went for number 3 (his 4th) and it was twins!
-The 1st wife died suddenly a few years later when his eldest was 20. He has special needs and moved in with his dad/my friend.

So within a few years, my friend went from 2 kids at home to 5. Her husband was happy to go for number 3- yours isn't happy to go for number 5 so its not an option.

if they were all your own i'd say you were being unreasonable but only one is, so it's quite different.
As an only child myself I very much get the wanting your child to grow up with a sibling. That said it's not like they wont have siblings, but they will be somewhat older and not so present, so its a different dynamic. While I think it's reasonable for you to want two children, your DH is also totally reasonable to not want a fifth. Environmental and financial impact is very signifficant. I really worry my (2) children not finding well paid jobs/being able to afford their own houses in the future, that would be multiplied for your DH in the future. Even though your step kids are currently favouring their mum's presumably that could change.
There are two different things here, your desire for another kid and is it really a good idea. These two things obviously conflict so its not a straightforward one at all.

I think YABU. Your partner isn't keen and even if he gave in, would he end up feeling resentful?
Also 39 is quite old to have a baby, as you know the risks are higher. I had a surprise baby at 40. Not only did I risk my life ( doctor said absolutely more children as it could kill you) but DS has severe special needs and will probably never live independently..
Obviously your baby could be fine but I want to make you aware.
Ultimately though is DP says no it's a no.

I think it's a selfish decision, personally, as it doesn't serve your current children at all – YOU wanting a sibling for your DD is still YOU. Your partner also not keen, so surely that's an absolute no.

Dedicate your time, money and energy to your current children.

@Eaglesfortea Why would you see this as good for your dc though? It’s a very low standard to go it alone because your partner is unreliable and walks out on his families. He’s a pretty poor dad to his existing dc and you want to compound it? Dc are very happy without a blood sibling. Keep the others close and encourage partner to see them. Is his former partner pleased to see the back of him? What’s the dynamic with his first family?

"forget about this environmental impact of children nonsense"

So you think exponential population growth can just happen indefinitely and somehow the planet will cope with housing and feeding everyone in future? Not to mention accommodating all their waste.

I think I wouldn't, on balance. DH needs to focus his attention on building up the relationship with his three, which sounds quite sad at the moment.

I wouldn't.
Your DP doesn't want one.
45 mins is really not that far and DP doesn't seem particularly bothered to make the effort with his first 3 DC.
DC are really expensive as older teens. You say you and DP earn well - he could be topping up as much as £10k per year per child at Uni if they decide to go London.
You'll be 58 at the youngest when the baby leaves for Uni and supporting them financially until you're 61/63.

Honestly your partner has already said he doesn't want a 5th child so for that reason alone it wouldn't be wise to have another one. Your other half would probably resent you if you went ahead knowing it isn't what he wants

How old is your dp.
On average The older the father the more potential problems for the child

That said it’s a two way decision

You don’t have four kids. You have one child.

Your husband hardly sees his other children.

Have your second now. This isn’t about your husband.

Was about to say yabu, especially on the environmental impact and stretching time and attention over so many kids. Then I read that you actually only have one bio child and hardly spend any time with step children anyway so I think neither of those are valid reasons not to have another one.

I wouldn't because he is your DP, not your DH unless you are the one who doesn't want to marry.