MJ82...you misunderstood.
I was distinguishing 4 in one sign from 4 in one house but not same sign.
The reason 4 in one sign is powerful is because the same element and mode is emphasized, an emphasis on the essential rather than circumstantial. Circumstances can be changed, while the essence of a thing (person) does not change.
And of course, if 4 planets are found in both one sign and one house, the stellium's power is enhanced.
Erulastiel: I really don't think it's a matter of personal preference. The definition of stellium that I offered is old; the "three planets within 5 degrees" is a modernist's re-definition of the stellium. Also, the planets in a stellium do not have to be in actual within-orb conjunction. A planet at 1 degree and a planet at 29 degrees with 2 more in between at any degrees of the sign constitute a stellium.
The reason a grouping of planets is called a "stellium" (or satellitium) -- which is a "formation" -- is due to its incredibly enhanced power, the power of concentration.
The stellium is a concentration both of Element and Quality (Mode). Three planets are not sufficient to produce this heavy imbalance of power.
It is the same idea as with Preponderance, of which there are many types: Element, Quality, House Triad, Cross, Retrogradation..... All of these preponderances require a majority of planets to qualify as a preponderance. A stellium is an exceptional preponderance, focalized in one place.
I submit that the authors who say three planets form a stellium are in error. Just because someone wrote a book does not mean they may not be mistaken. And then what happens, if indeed they are in error, is that students and followers come behind them and propagate the error.
A stellium is composed of 4 or more planets, at least 2 of which are not Sun, Mercury or Venus.