Re: WWYD - 4 year old reaches RF weight limit
I would give myself a big pat on the back for making it to 4 rear facing and I would pick out a combination seat.
Just so you know, the ProSport is not narrow. It does have flat sides that don't flare out, but it's pretty wide. You could also try the Evenflo SecureKid 300 or 400 or the Nautilus or Frontier to see if they puzzle with the other seat. If you can't get a seat to fit in that seating position, he will be safe outboard in a properly installed, properly used seat.
CPST-I and SAHM to DS