I have 3 in a 2003 Corolla. Ages now are 10, 8, 5, but the 2 oldest are still in boosters and youngest in a 5 point harness. I've had this car for 16 years, so before they were born. Once the 3rd came around the older boys were almost 5 & 3, so we graduated the older one to a booster around then and put him in the middle w/the 5 point harnesses on the outsides, but only seldomly used the Corolla as we had a mini-van and didn't go around in my Corolla together. To add a problem, my Corolla is manual and my wife never wanted to learn, so we can't just trade cars off. Now I have the youngest in a 5 point harness on the side, the middle is a bumblebum and outside is a normal base booster. It took them getting used to to move the seats around to buckle in, but just like anything else, they've learned what they need to do after some frustration and now we're good to go. My 10 year old probably doesn't need the booster anymore and within a few years he'll be in the front seat, so we'll be okay. If your oldest are twin 4 year olds and will be close to 5 when your new baby is born, that really doesn't leave a ton of time before they switch to boosters and then at that point it will probably be pretty easy. So I would keep the same car if I were you, do the family driving in the other car, or if you have to divide and concur just get an extra bumblebum car seat and throw it on the floor so you can use it in a bind.