That's right. 4.6 million Guardians in the beta, on all platforms, according to Eric Hirshberg, that surprisingly handsome chap from some of the videos and ActiVision Blizzard Publishing CEO.
This makes it apparently the biggest beta on 'this console generation by a wide margin', a confusing term given how Destiny is a cross-gen title. That said, the 8th generation has begun, and considering the first console beta ever was H3 in the last generation, then yeah, 8th gen consoles.
It was also the biggest beta for any new IP ever. Nice work, Bungie. :D
Edit: the H3 beta was the first ever beta I'd ever heard of on console, as well as the first I ever played, but maybe it wasn't the first public one overall. I don't know. Google isn't being very helpful either.