The short form is that no one has access to the numbers really needed to prove that beyond the shadow of a doubt. There are two bits of data we do have to indicate relative sales, both of which are flawed.
- ICv2 Magazine surveys the hobby sales channel, and frequently reports the top 5 RPG sellers in order - but there is no metric in their rankings to indicate by how much. Also the hobby channel numbers (which are themselves based on surveys of distributors and not anyone's real balance sheets) omit the book channel, Paizo subscription sales, D&D Insider subscriptions, etc. ICv2 does opine based on discussions with store owners (unclear how many and how representative a sample) that "As far as the publishing side, Paizo’s Pathfinder line appears to have taken a huge chunk of market share away from D&D, with many stores reporting it supplanting 4th edition as their number one seller." Here's the historical tracking to update as we go...
- Amazon sales ranks, which are only specific product by specific product and only based on the last 24 hours, sometimes show Pathfinder outselling 4e but that's not very reliable.
Over time, you can follow the ICv2 and Amazon numbers for what they're worth, both are updated over time so this should be as valid a methodology in the future.
Neither WotC nor Paizo have been forthcoming with any sales data that would nail any of this down (some game publishers do release this info, but neither one of them does).
In the end, it shouldn't matter that much. It is nice for Pathfinder fans to see that their game isn't a fringe game but is rivaling the largest in the business, it provides reasonable expectation of continued viability.