Wow! I know this is AIBU but I can't get over some of the responses.
It's a tough one OP. I think I posted a similar question a year or two ago and, to be fair, I realised that when the posters didn't know me or my life or children or circumstances, its a difficult question to advise on.
People have completely different levels of resilience - I know a woman with 7 children who takes it all in her stride and I know another who openly admits her 1 child has broken her and she is exhausted. It's hard to know what to say without knowing you. You know you're own limits.
I have 3 by the way and am currently pregnant with number 4.
I'm sure there's plenty who take I'm insane. It took a long time for me to come to the decision (my youngest is almost 8!) and I over-thought the absolute shit out of it. Now that I'm pregnant, I'm very very happy with our decision.
Our children are ecstatic.
I'm going to come from a very different perspective to many on here: I'm not very convinced that having more children impacts negatively on the current children as some have implied.
I am an only child.
I can't tell you how much I hate being an only child. Its worse since I've gotten older. My parents are my responsibility (well my mum is, my dads dead now): my sole responsibility.
My gran recently had a stroke and its been awful, but its also been lovely to see how her 9 children have pulled together to support each other practically and emotionally (and share the financial burden) in relation to her care.
When my mum is elderly, I will take care of her as needed but it would be lovely to have someone share the responsibility.
When my dad died, there was no one who 'got it', there was no one to share the grieving process with.
I feel very alone as an only child.
Even now - even though I'm a big and bold 33 year old woman.
In those extended family arguments, there's no one in my corner.
You're forever alone (well until your own children are adults, I suppose).
I could sit here and go on and on and I understand that I clearly have issues but I love that my children will never experience these things that I've experienced.
Obviously, when they're older two of them may just not get on, but there will always be another sibling to turn to.
I think large families are gorgeous and I envy them and now I have one of my own.
We'll see how life with 4 is first obviously but we have certainly not ruled out a 5th.
Go with your gut, I reckon.
The best of luck to you 