It totally depends on the drive train of the RDX. Honda (Acura) stopped certifying the Pilot (MDX) for the 2006 and newer models. HOWEVER, the drivetrain did not change. Honda only got tired of providing warranty work for idiots how didn't follow the set-up instructions exactly. So, if the RDX has the same transmission as the Pilot and MDX, then yes, it can be towed be towed 4 down.
But, (there's always a but, right?) You take full responsibility if you don't follow the set-up rules precisely. Many people are towing their Pilot, Odyssey up to model year 2013 IIRC with no problems whatsoever. After that, the transmission change is when 4 down towing became no more.
I bought a 2005 Pilot last year and wish I had done more research, I would have bought a few years newer model if I had of known what model years had what transmission.
Hope this helps,
Tom