Well, if you think of each PHB as a "new content adding patch", then I think the essentials would be 4.3.numbers (4.3.1.numbers for the "first" Essentials book, 4.3.2.numbers for the "second" and so on).
PHB being 4.0.1.numbers, MM being 4.0.2.numbers, and DMG being 4.0.3.numbers (MM and DMG...