So if the PCs engage monsters in room 1A, and monsters from room 1B join the fight, how much difficulty they add to the encounter will mostly be a function of it's level and minion status.
Of course, difficulty will vary from party to party. Party's with more co-operation have a tendency to do better with difficult combats. This was true in both 3.x and 4e. (I suspect this is true for earlier editions as well, but I haven't played them enough yet.)
That is true, with the addition of environment and situation to level and minion status on the difficulty of the added monsters.
However, I managed to obscure my main point with the example: I think it highly unlikely that someone could devise a 4E encounter for, say, five to six reasonably competently played PCs, play it 20 times with essentially the same tactics, and get results similar to my 1st ed. example above. If this hypothetical deadly 4E encounter will TPK the party somewhere north of 50% of the tries, the chances that the group will manage to pull out a smashing victory are vanishingly small. Chances for a non-TPK retreat, or even a partial victory are there. But outright victory? No.
It isn't surprising, since 4E is designed to produce this result. In part, it does this by having more rolls, thus reducing the chances of a string of 1s or 20s being all determinative. In 1E or Basic, a string of 3 20s or 3 1s at the right moment could be the difference in "Smells like Victory" versus "Time to roll up some new guys." In 4E, such a string is more like a difference between "Smells like Victory" verus "Had to use another daily", OR "Pulled out all the stops in desperation and squeaked by" versus "Time to stat up some new guys". I'm not sure of the exact number, but I think you'd need a string somewhere around 6-8 such rolls (good or bad) to swing a 4E encounter into as wide of a likely range as 1E.
Caveat: Because 4E is so team focused in its tactical combat, it is true that a few bad or good rolls at the wrong moment can have a domino effect that, in hindsight, will be seen to have turned the fight. Getting "Leader of the Pack" off, or not, from a Tactical Warlord, is one roll that can do so. However, the many rolls before or after it will still have significant say in the matter, making this a somewhat different dynamic.
This affects perception of how ordinary or amazing the characters are. Played more or less according to the book, a party of 1E characters that has made it to around 7th level are "superheroes"
because they have made it. All those "ordinary" characters you played that TPK'd at 1st level, 30 minutes after play started? They are a big part of the character pool, but a relatively small part of the play experience. In some ways, they are color--part of the universe in the same way as the Travellor characters that died during char gen. It is merely that the D&D "char gen" was a combination of rolling up the character and running through those first few encounters.
