I have 4, aged 9, 6, 3 and 1. I agree that there isn't much change from 3 to 4 in terms of being able to cope with lots of children etc but I think ther eARE some differences that I've noticed.
1- logistics. You can't get all of you in a normal car, obviously. But it is a pain having to attempt to hire MPVs etc everytime you go on holiday because they either don't have them or they cost a fortune. It also means that anyone else that you may want to lug your kids around (eg your parents) will need a car that seats all of them. It is just a bit of a pain.
2 - holidays. going anywhere, doing anything, costs a fortune. And hotels are tricky, because you HAVE to have interconnecting rooms and you HAVE to have at least one child with you at all times. Somehow with 3 children you can even almost get away with one room sometimes (eg if there are 2 double beds and a cot or rollaway) but with 4 forget it.
3 - juggling. there just is a lot more of it. as the older ones get older and need to be in different places at different times, you begin to think that you are a taxi service, and you start feeling really sorry for the little one who spends his/her life in the car. And you DO look at people with 2 kids and think "your life is just a piece of piss"
4 - school. it will go on FOREVER. you will have an extra X years of school runs, you'll have to do biffkipperchipfluffy AGAIN, it is another round of school plays, sports days, geography projects etc etc etc. Good fun the first time, OK the second, boring the third, by the fourth you JUST DON'T CARE about any of it as long as he/she goes in the morning, comes back in the afternoon and doesn't take drugs in the middle. Or at least that's what I can see from here, I only have 3 at school so far!
5 - sleep. I don't know about you, but after basically 10 years of broken nights due to pregnancy, baby, toddler, pregnancy, baby, toddler, pregnancy, baby, toddler, pregnancy, baby, toddler I'm about ready for some late-sleeping teenagers. Only 12 years to go then
So all in all it is great, but a little bit grinding and knackering and repetitive. My dh looked at me at 5 am on Sunday morning when the little one woke up and said "do you think she is the straw that broke the camel's back?!"
He was joking, but only partly!
Good luck