We like the boosters where it's just a base and the "back" can detach on it.
Graco and other companies make them, and they are sold EVERYWHERE including Target and Wal-mart. Graco calls theirs the Highback Turbo Booster. Everyone we know and carpool with uses these.
With your son already being 4.5 it sounds to me like he's outgrowing the pricer ones that are full on carseats, not just boosters.
Since your into Britax, and this isn't going to be his primary carseat, but the secondary in Dad's car what about this one: http://www.amazon.com/Britax-Parkway-Booster-Seat-Onyx/dp...
Incidentally, I am a HUGE safety freak too. And we went Britax for our kids until they outgrew their infant/toddler carseats. We did move to the Graco boosters and LOVE them. They are well made, easy to clean, easy to move and the kids have been very comfortable.
Good luck!