I would check out the Sunshine Radians. I do not have them, but my family is in the process of buying a smaller, more fuel efficient car for commuting, and we would love to be able to fit all three of our kids into both the mew car and our minivan, which we will continue to use for trips, etc.
Anyway, I am responding to this more to say, be careful about advice to simply put the 4-year-old into a booster. Boosters require kids to be 40" tall and at least 40 pounds. My 5 1/2 year old only weighs 37 pounds, and so does my 4-year-old. I know I won't be comfortable with either one being in a booster until they are at least 41 or 42 pounds to ensure they are as safe as they can be in the booster. It might cost you more to have to buy new carseats, and heaven knows those Britax seats aren't cheap (we have 5 marathons -- that's $1,250 worth of carseats!!), and it will be frustrating to have to replace them. But I gather you are safety conscious because you have the highly rated Britax seats. If your child meets the requirements for a booster, great, go for it. If not, I would err with your safety consciousness and go for the new, more narrow seats. Buy just the two you need now because your 4 year old may well be ready for a booster by the time your newest one is done in the infant "bucket."
Good luck with the new baby. Being a mom of three is a busy trip -- but it is a trip worth taking!