For something like this, they should just call it 4.25 D&D outright.
PHB1 = 4.0
PHB1 + PHB2 = 4.1
PHB1 + PHB2 + PHB3 = 4.2
The monster manuals are far worse.
MM1 = 4.0beta
MM2 = 4.0 (feels like what the MM should have been)
MM3 = 4.5
MM1 is so far behind now unfortunately that this is basically true. When I wrote my other post just now about what needed to be changed to make those monsters "up to date", I really realized just how far the design had come. That's not a criticism incidentally of 4E, I think it's fantastic the game is continuing to improve - but we really are leaving a lot of iconic monsters like chromatic dragons wallowing in mathematical mediocrity.