B.K.
My almost 4 year old son has been in his Marathon since 10 months of age. He won't be growing out of it any time soon either. 65 lb weight limit and 49" height. A 4 yr old is usually about 35-40 lbs and 35" or so. Plenty of room left. I have been extremely pleased with it. It is also incredibly easy to install with latch or the seatbelt. My parents have even installed it in their non-latch cars w/o my help or instrutions. Britax is the best in terms of ease of installation. I just purchased a second Marathon for my 8 month old daughter. They sit high off the seat so your child will get a good view out of the window. They are also appear to be very comfortable. My son sleeps very well in it. Big, so not easy to travel with but it fit forward and backwards just fine in my little Acura TL (in the center seat). I'm in an Honda Pilot now that I have two kids. We'll see how my daughter's fits backwards behind the passanger seat. Haven't put her in yet but I'm sure the passenger will have to ride a little closer to the front than I would prefer. They are great, you won't be sorry. babyage.com had them on sale for $220 + $8 shipping yesterday. Good luck. Lastly, the Radian's caught my eye too. I considered them for my daughter. They are made of steel but I didn't like the fact that they sat lower than the Britax and I read they are more difficult to install. I just figured I'd stick with what I love. The Evenflo Triumph got decent reviews too, if you are concerned with $. Not as great as the Marathon and Radian, however. Good luck!