forgive me for posting when I do not have a large family myself, so not speaking from personal experience (I've got 2, am curious about 3+), but the following thoughts spring to mind, you may have thought these yourself already. By the way, you don't have to answer these on here, but you might want to ask yourself:
what does DH think? How much do you rely on him for support? (practical and emotional)
what do you think would be the impact on your other 3? short term and long term? Time, money etc
can you afford it? I personally wouldn't be keen on the small house bit of it, especially if you are unlikely to move somewhere bigger/extend anytime soon.
Are you bothered about your career/hobbies making a come back anytime soon?
Quite a lot of what you said sounds more on the negative side (laundry, tantrums etc). I would have thought the laundry and fighting between siblings and also school run will not particulary get easier, if anything may be harder when DC2 and then DC3 starts pre-school and DC1 and then DC2 start doing clubs/play dates etc
The odds of complications in preganancy are greater the older you are (not that you're old, but obviously older than when you had your first) - this is one that people don't like facing up to but it is true unfortunately.
HOWEVER:
A lot of people have difficulty shaking off the idea of x number of children, if that's what they have in their mind and I really admire those people who just go for it. It usually works out fine. Children are a gift and so are siblings.
I think in many ways having a big family is like a small one, but you get more ups and more downs, i.e when it's good it's amazing and when it's not it's really not!
What are you like regarding other decisions? Heart or head? Although, trouble is having a baby is not really like any other decision you make... 
It would be good for someone with 4 older children close together to come on and answer some of your other questions. I kind of think it's one thing having 4 under 6 but what about having (say) a 5yr old, 7yr old, 8yr old, 10 yr old. That sounds like a bunch of kids who still need a LOT of time and attention, not to mention socks washing and new school shoes, trainers etc every 6 months!
Sorry, gone on a bit there and I'm sure those who have 4 or more can be a bit more encouraging (maybe that's why you posted on this board?) 