@kaleandcoffee Four isn’t a huge change from three. The adjustment was much easier for us… but there still are costs (in money, energy, attention and so on).
However, my advice in general is only have another if you and your partner want to raise another, for their own sake and not solely for the benefit of siblings (although siblings can be wonderful friends, shared parentage/raising doesn’t mean kids will like each other or get along). Your youngest could always rely more on friends outside the nuclear family for companionship - many children do and are perfectly content/fulfilled.
If you have the energy, ability, and desire to care for and support another kid, awesome. If not, it’s not worth it just for a “matching set.”
I was 5/6 and have almost no meaningful relationship with most of my siblings as an adult. My husband was an only child and has dear friends who are closer than many brothers. We have a pack of our own, but never with the expectation that they would necessarily be BFFs (though it’s looking likely for many of them…)