Can you elaborate on the Radian's positions? My manual says nothing....
But anyway, the bottom three are fine for the Scenera...if you were using the top one then your kiddo's head would be too close to the top of the seat to be rearfacing safely (which is actually the same rule I use for the RN, my dd's been at the second to top slots for months now, rearfacing, head well below the top of the seatback).
Edit: Oh, here's the transcript of the chat with the carseat engineer about the Radian:
"Carseatengr -> Special Events - CEU Chat Session: Re slots rear facing - if one follows our weight limitations regarding rear facing any slot may be used.
Carseatengr -> Special Events - CEU Chat Session: And I might add our instructions regarding the harnesses must be followed as well - so if the rear facing child is tall and is 32 lbs it is possible the 4th position could be used. "