Incgamers recently made a post about it that shows the actual bug in action. There's a direct link to the video here and you can see it happen immediately at the 2:03:22 mark.
I wanted to point out something interesting about this "bug" that most players might not have noticed or understood about video game logic.
Did you notice that there's a 5th invisible portrait window already in the game? If this were a bug, why are there logical in-game systems in place to support the addition of a 5th player to the party? Not only that, but you can interact with this portrait window and see player information.If this were truly a bug, and the game was not designed for 4+ player parties, none of this would be possible.
A fifth person could join the game, but there shouldn't be a fifth portrait window prepared for this user and there shouldn't be other logical systems in place to support more than 4 players. What this means is that the interface was designed for supporting a minimum of 5 players. And, I say "minimum" because there's still plenty of space (and small scaling of player portraits) on the left-hand side to support more than 5 "invisible portrait boxes". :)
This isn't a "5th player bug".. it's a bug in their player number restriction system, and players are able to bypass it to get more players (which are actually supported by the game.) Much like the public channels (which have been probably in the game for over a year now), it's really just an on/off switch that Blizzard controls. The game is designed to support more than 4 players, but Blizzard has it switched off for some reason.
It could be because of console limitations or their design plans for co-op, perhaps they were originally planning on 5 for the current game or as part of the future Diablo 3 X1 expansion (group increases in size with the introduction of new classes), or maybe the developers had problems scaling the monsters so it was easier just to make it 4 players. I think that a lot of players believe the latter.. that it was for scaling purposes.
So, something occurred to me. What if the game didn't just support a 5th player portrait window, but also other in-game systems already.. like scaling?
Guess what? It does.
When a fifth player joins the party, you will actually receive an alert ("A new ally has joined, but the minions of Hell grow stronger") and the monsters properly scale/grow in power with the addition of a fifth player. (There are also other windows/UIs beyond the party menu that demonstrate the in-game logic for supporting 5+ total players.)
So, adding a 5th player is not a bug. The only bug is that players can bypass the security lockout that prevents the 5th player from joining.
But, it's interesting to know that the game was designed to support a minimum of 5 players, the current game fully supports it, and monsters will scale when the 5th player joins.
This begs the question; why is there a 4 player limit? It's not due to scaling issues, it's definitely not an infrastructure or processing problem, the game was DESIGNED to support more than 4 players, it's not for testing purposes, and it's not revenue driven (ie, Blizzard wouldn't create a paid service that "upgrades" the player limit.) Blizzard specifically picked 4 for some reason, even though the game (and gameplay) supports 5 and more. They apparently don't plan on unlocking this (since it's "a bug") for a future Expansion Pack.. so why did they select 4?
Although highly unlikely (according to Blizzard), if something in the future is planned, perhaps the 5+ party isn't for "normal parties" but rather a support mechanism for special dungeons (raids) or large scale PVP combat teams.
And here's something else to think about: If this bug can be reproduced in the retail (which players will probably find a way), what kind of exploitation potential will there be for 5-8 player Inferno parties where the monsters can be killed more quickly by MF-geared players. (Apparently, loot drops don't scale with the mobs.. however loot scaling has been both confirmed and denied by Blizzard.)
* UPDATE:
Apparently, back in August 2011, Jay Wilson confirmed that the game was designed for 6 players maximum, but they didn't like the chaos of having 5-6 players on the screen at once, so they were "playing around with 4" which they believe would be the eventual maximum party size.
So, as suspected, the game was designed for at least 5 players (6 being the programming logic cap), it has the math to support it, and it appears that rather than going back and reworking a lot of the code (which would take a lot of time), they simply locked the max party size to 4, even though the game supports more.
Considering this, it is entirely possible that users might find a way to get at least 6 players in a party (in the beta, but hopefully they won't close all of the gaps by retail.)