I have 4 ages 8 (girl), 6 (boy), 4 (boy) and 20 months (boy) and I LOVE IT! I agree with the pros and cons...they're each others best and worst (at times) friends and in the beginning they'll enjoy the same things but as they get older they'll each find their own thing. I have't really reached that point since they are still fairly young, but I know its coming. And I know my weekends will never be the same, but I wouldn't have it any other way.
And be prepared for the strange looks and comments...I've heard so many that now I just smile and laugh. A few are "Oh, you poor thing!", "Don't you guys have cable?", "Are you done NOW?", "Did you plan them all?", "You're a busy lady.", "How can you afford it?" and the list goes on and on.
Financially...you'll adjust...we did, but we're also fortunate and blessed to have my grandma watch the two younger ones while the older two are in school so we don't have the child care expense. When we have family days out we usually go to the park and have a picnic. We've become creative with finding things to do that don't cost a whole lot-it's makes for a fun adventure and quality, not quantity time.
For your new 3 questions:
1. I took naps with them whenever I could. I know you may want to do housework or laundry or even play with the other two, but you have to let those things go. (They will ALWAYS be there unfortunately!) And don't be afraid to accept help.
2. My two boys slept in the same room with no problem. They have twin beds because I didn't want them jumping off a top bunk! It actually works out well. I think having each other in the room they're not afraid to be alone, but it does sometimes lead to playing around, but they're kids, you have to expect that.
3. When I had the three older ones I had 2 car seats and an infant carrier in our Tahoe. They all fit without the infant carrier base. Now its 2 boosters and a car seat since my girl is old enough to be without a booster.
Hope this helps! Good luck ;)