remember that one important aspect of all "gaming crack" stuff is not just collectibility but also POWER INFLATION...
...so, 4E might feature:
level-linked books - you want to level up? Buy more books!
e.g.: DMG4E, D&D4E-Rulebook, PHB4E-Level1-5, PHB4E-Level6-10, PHB4E-Level11-15...