J.O.
We did 3 across in a smaller car BUT we had two Dionos (5 pt. harness seat fits most the kids age 6) and an infant seat or even a convertible. The Radians are very 3 across friendly. My son was 50 inches when he switched to a booster. I don't know how tall your 6.5-year-old is, but for the 4-year-old I imagine the Radian would work. A 4-year-old in a booster would make me a bit nervous. Plus you have a child to pass it down to.
I also know from experience that buckling two boosters in a tight spot is very difficult! You might go crazy. :) So if you can go at least one harnessed seat, baby seat, and just the one booster, for now that might be easier.