This is my fourth, too, but I planned for a big family. I have only a four bedroom house, and my parents live in one, but the bedrooms are huge - big enough to house 10 kids between the two that I currently use for my three (who are 3, 2 and 1). I'm still in my 20s and come from a family where you're fertile for a long time and have huge families - average 13 kids. That isn't my plan, but my house is built around having a big, close family because I always have friends or relatives over.
I live in the U.S. and have a Honda Oddessey minivan that seats 8 and currently houses 2 rear facing carseats and one front facing. When my nephew comes around, we use another front facing carseat for him. I can fit 6 car seats (rear faced, front faced, doesn't matter) and 2 adults (mom and dad) in my current car, but I am thinking of getting a bigger car that seats 12 (a Mercedes Sprinter) because my parents are elderly and ride in my car a lot. We also need more "trunk" space for double strollers and bikes and such.
I have lots and lots of living space in my house - it's over 4,500 sq feet. There are 3 family rooms (one is currently used as a music room), 2 dining rooms (with 18 chairs between them right now), big hallways, an office I can convert to whatever, a finished basement, 4 bathrooms (3 with a shower/bath), etc. but like I said, I prepared for this life. The house I lived in before this would only have held 2 kids in bunk beds, so I'd be in trouble already with 3 kids. We also just bought our van when we found out we were expecting our third.