I have a group that has recently been playing with three PCs, and while it is possible to optimise hard and have some chance in the published modules, I think it's fair to say that the group is simply too fragile. Dropping the numbers in the opposition was making for kinda odd encounters, so I started wondering what it would take to allow a 3 PC team have a solid chance of playing against standard encounters.
Then I thought, gestalt rules should be much simpler in 4e. The power system makes them the classes mesh well. For all I know this has been examined before, but here are my suggested rules:
Rules:
Gestalt characters take two classes at first level, and take all class features of both classes. Any defence bonuses from the classes stack. They gain all options to take powers, feats, paragon paths and epic destinies as though they were a member of either class. They may not take multiclass feats of either of their base classes.
Hitpoints: Gestalt characters gain the largest number of starting hitpoints, healing surges, and hitpoints per level of their two base classes.
Skills: Gestalt characters gain all fixed skills from their base classes, and the largest number of freely chosen skills from their base classes. Their class skill list is the union of the skill lists of their base classes.
At-will Powers: Gestalt characters have the normal number of at-will attacks of one of their base classes, and an additional at-will power from the other base class. Humans may choose their additional at-will power from either of their base classes. Half-elves may not choose their dilettante power from the base class in which they gained their normal number of at-will attacks, but may freely choose from all other classes, including their second base class.
Powers: Other powers are gained as normal, but from the combined power list of both base classes.
How strong are these gestalt characters?
They're still limited to the same number of non-at-will powers, and most importantly, they're still heavily limited by their choice of stats. The power creep is via the class features, a little by the ability to pick and choose powers, and (if the MAD problems are worked out very carefully) the ability to play multiple roles as circumstances dictate.
I would say that a team of three gestalt characters would still be very much weaker than a normal party of five. They might start to rival four normal PCs, however. Even that is strongly debatable. Offensively, the total pool of actions, encounter and daily powers and focused fire is just that much smaller. The numerical differences of more optimal choices would be hard pressed to compensate. Defensively, the reduced number of targets and total hitpoints make the group more fragile, as do the reduced tactical options. Perhaps 4 gestalt ~ 5 normal PCs.