Imaro
Legend
I quoted p15 of the 4e PHB above. Here it is again:
The classic adventuring party includes one character of each role: wizard, fighter, cleric, and rogue.4e is not geared towards a certain party composition. In fact, I would say it is very robust across a wide range of party compositions. Part of the logic of its flexible PC buid rules, retraining rules, magic item wishlists, etc is to put a good deal of the onus on the players to build PCs that they find mechanically satisfying.
Character roles identify which classes can stand in for each other. For example, if you don’t have a cleric in your party, a warlord serves just as well in the leader role.
Roles also serve as handy tools for building adventuring parties. It’s a good idea to cover each role with at least one character. . . If you don’t have all the roles covered, that’s okay too—it just means that the characters need to compensate for the missing function.
That said, the DMG does give good advice on the mechanical side of party building, including how different sorts of terrain affect different PC roles. This is one respect in which the 4e DMG is not at all deficient.
Emphasis mine... I would claim pg.10 in the DMG seems to contradict you on this point... you know where it cautions against not filling the four roles and gives the DM advice on how missing roles will affect encounters.