My daughter will be 4 in Feb. she is currently 38.5" tall and 34 lbs. She is in a Radian and my son is rearfacing in a Marathon. We are going from a van to a 5 passenger vehicle and need to be able to have a third person in the backseat between the carseats (an adult). Originally, we were going to pass the Radian to my son and get my daughter the Graco nautilus or Britax Frontier, however, we now have a space issue. Are there any seats other than the Radian (in Canada) that are narrower than the marathon (harness to booster?). If not, how long would a Radian be likely to fit her harnessed? We are thinking, at this point, that it may be our only option. I would like to keep her harnessed as long as possible.