ICv2, given its limited scope and methodology, already doesn't mean much. It serves only as a flawed measure of interest in particular games.
And, in that sense, I think the comparison could be quite interesting.
Yeah, on Amazon right now, for example, the 4E PHB is outselling the current Pathfinder rulebooks (and PHB2 over all but one).
The fact is, we really don't have a good view of what/how things are selling, regardless of what particular metric edition warriors are touting as "proof". The companies just aren't letting us know (nor are they obligated to).