Do you mean that you're considering TTC now when you have a 4 month old? Oof, no thank you, not for me.
I'm in my early 40s and our 3 children are teens/adults and I couldn't imagine going back to those baby days when we're experiencing the freedom that comes without very young children but also getting to enjoy the fun things with our older children.
Appreciate you've already dived into that with your 4 month old but even when I was in my 20s with a baby, I couldn't have chosen to try for another that could potentially give only just over a year's age gap. At 4 months just one baby was quite enough with breastfeeding and all the other care and developmental support that tiny human needs.
Having twins or 2 under 2 sounds so incredibly hard. It sounds like your youngest has been a dreamy easy baby but what if another baby isn't? What if your current youngest sleep regresses or is a really challenging 3 year old?
It's not impossible, many have done it, but would I choose it? For my own mental health, emotional and physical well being it would be a strong nope.
Let your body (and mind) fully heal post birth.
It sounds like your guy is a good dad and a good step dad. But that's not enough to rush into anything. I understand the time pressures of TTC at our age but I also think it's OK to enjoy the children you already have. If another child comes along, fine but I would have struggled massively with such a small age gap. You're looking at double buggies, two big car seats, two in nappies, double nursery fees (although it sounds like you're wealthy enough that any of the financial pressures of parenthood aren't an issue).