I am 44, and have 3 kids 9,7,5, also had 3 under 4. I sometimes hanker after a 4th, but have friends who've gone back to the baby days when their youngest is at school, and it massively changes life.
I love tiny babies, but for me, going back to the crawling, non-sleeping, into everything phase would probably lose the few marbles I have left.
Only you can answer it, but I'd make a long list of all the differences it would make to your lives; financial (nursery etc), logistical (doing all the club runs with a baby in tow), do you have space for a 4th in your car or would you have to buy a 7-seater, where would no.4 sleep and would it mean someone has to share that doesn't currently, and finally emotional - what would your children think, how would it curtail or enhance their lives, and the same for you and your OH.
If the answers to all these are positive, and you feel you have the stamina, go for it! For us, the answers are largely negative and therefore we won't be doing it. Oh, and also bear in mind that the chances of having a multiple birth increase with age, if you are close to peri menopause your ovaries get trigger happy and release multiple eggs in one go...