I had three by IVF, and would have liked to have 4, but the funny thing is that wanting the fourth didn't really happen until it was too late ...I remember thinking I couldn't possibly cope with 4 when I had the three under 6, and relieved up until they were 10, and then a bit regretful until they were 16 (the youngest I mean) and then it hits you, I want another and it is too late!!!
So, warning, if you possibly possibly want another, you will probably cope even if you think you won't, but if you don't have the fourth, you will never ever know...(or the second or the third) There's no harm in trying.
Still, 3 is absolutely lovely and I'm very blessed to have 3, as dh is always telling me. I think men are quite reluctant to have more than 3, they can colour our approach. I think that men think in terms of all the trouble and women can sometimes just be very broody, despite all reason suggesting that you don't need more.