When 4.0 was released to the public, the next update provided 40 b13 nightly users was a 4.2 alpha. (some plug-ins break)
They are moving to a Google Chrome like development schedule. So yes, basically, 4.0 is stable, 5.0 would be beta, and 6.0 would be alpha. Soon, 5.0 will be stable, 6.0 would be beta, and 7.0 would be alpha.