If you're looking at the Radian, be aware that the top slots for the shoulder harness are the same for both the 65 and the 80. It's a better buy to get the 65 (save yourself almost $100), because your 4 year old will outgrow both at the same time because of height. Also, the Radian has had many vehicle compatibility issues... it's tough to get a secure fit. Buy one where you can return it without hassles, so that if it doesn't fit, you're not stuck with a $245-75 seat (if you get the Radian 80).
I purchased the Radian 80, so I speak from experience. It fit nicely in our 99 (latchless) Accord, but when our little guy (also 3.5) got in, he is almost to the top harness, so it won't last more than a year for him. I had ordered it on-line from Target, and am returning it to the store today. Keep that in mind when purchasing! Also, Babies R Us (in Orland Park) doesn't carry the Radian... at least not as recently as two weeks ago (2/29/08). Target does not carry the Radian in-house, only on-line.
We purchased a Regent for our CRV. It has the highest shoulder slots for a five point harness currently on the market, so it will take a lot longer for your little one to outgrow it. It is a huge seat though, so you also want to make sure it will fit nicely along with your baby's seat in your vehicle. I ordered it from babycatalog.com and purchased the $5 return shipping guarantee, that way you don't get assessed a $50 return shipping fee if it doesn't work out. BrU does carry it, and you can try it out in your car to make sure it'll fit. Also Target does carry it both in-house and on-line.
FYI Britax does do at least one (sometimes two) big sales a year, for one week, with huge discounts for their seats. The Spring one just ended (March 2nd), but I got my Regent for $204, no tax, no shipping and handling. They sometimes also hold a sale in September... Just in case you have time to wait.
As for the Accord, we are off to Walmart to purchase the Graco Nautilus.
Also, if you can hold off, Britax is coming out with a new combination seat with a 5 pt harness and high weight limit. Don't know much about it, but it's supposedly due out sometime between April and September.
Whatever the case, and one of the other moms already referenced it, but a booster (although with legal guidelines of Illinois) is not the safest place for your toddler/pre-schooler to be... a 5-pt harness is. Likewise, your baby is best kept rear-facing until he/she is 35 pounds.
If you need a new seat for your littlest guy, consider the Radian, he/she'll get lots of use, and it's rear facing as well as forward facing... and designed for little guys!
Good luck! I'm walking the same path right now. Just about to schedule an appointment with the car seat installer for the Illinois State Police (based in Ashkum), she came up to Bradley and put the Radian in for us there. Now we've gotta get that seat out, and replace it with the Nautilus, and get the Regent installed. I'd love to wait for the Frontier, but I don't have the luxury. He's 40 " tall (but safe with the shoulder straps in his current seat) and 38 pounds!
L.