Do all porsche 911 models seat 4 people?

No, but the exceptions are just a few and limited to special editions of the 911 series (assuming, of course, you are referring to regular production road models, not race cars). An example is the 1989 911 Speedster edition.

Also, it should be clarified that the 911 was produced from 1965 and 1989 (model year). The versions that followed are commonly referred to as "911" but in reality they are different series.

Finally, on a side note, while most of them have rear seats, to say that they seat 4 people is quite optimistic...

LOL ... take a look at the image Motorhead has included to see what these "seats" look like. Still, I recall squeezing into a similarly equipped Porsche 356C (predecessor to the 911) for a ride to high school many mornings with my friend's dad. If the people in front have long legs, the ones in back must have no legs. Car and Driver once said, "The rear seats are suitable only for the punishment of small children."

Some of the GT cars don't have the seats installed, to save weight. Still, there are carpeted spaces back there you could inhabit, sans seat belts. There have been options replacing the seats with storage boxes that opened like big gloveboxes.

All of the cars, right through the current 997 series, which will be replaced in 2012 (998?), ARE 911s. The 40th anniversary edition (a 996 series car sold beginning late 2003 in the U.S.), even said "911" right on the tail - http://cars.ign.com/articles/432/432700p1.html . The only sports car which has been produced longer than the 911 is the Corvette. Mustang is third in this ranking.

All 911s (other than the GT2 and GT3) have rear seats that little kids can fit into, but not much else. The cayenne is a full sized SUV which seats a small army of adults. Can definitely be used as a family car. several others over the years have also had 4 seats (like the 928), but I wont bore you with the details.

Just about all 911s have these things you might call rear seats. You might use those if you have 2 small children

Not all 911s have rear seats. In the current lineup the GT3RS doesn't have seats at the back, just a reinforced "rollcage" of sorts.

Then again, the rear "seats" on present 911s weren't meant to stow away people. They were meant as additional luggage space

Here is an image of the undersized rear seats of a typical 911:

My 1967 911S had two folding jump seats in the back. Too small for adults, but excellent for kids. The noise level at Autobahn speeds was horrific. The jump seats when folded flat could accommodate one medium sized suitcase.

No, for example, the 1976 Porsche 911 does not.

the only porsche that has four seats but it has four dorrs is the panamera