I've always thought l4d series is valve made, guess not
Only 5 person per multiplayer match is disappointing though
I hope this helps:
Turtle Rock Studios was founded by Michael Booth in 2002 and did contract work for Valve for a long time.
They did the
Xbox port of Counter-Strike, the final version of
Counter-Strike: Condition Zero (largerly based on the the Xbox port) and they did
mapping for Counter-Strike: Source. They might have contributed to other MP games of Valve as well. They were Valve's go to MP contract guys.
One of Booth's experiments was a CS mod named
Terror Strike where people were fighting bots that had knives only. Out of that idea
Left 4 Dead was born and Valve agreed to produce the project, as usual (only this time the idea, eg "make a CS", wasn't theirs, but Turtle Rock's).
While Valve did not develop the game, they were in the loop with what was going on, played builds, gave feedback etc, as producers.
10 months before the release of the game, Valve made the offer to Turtle Rock to become a part of Valve, as they had been external contractors for Valve for 6 years now and had never made a single non-Valve game. So why not make it official?
And so it happened, and Turtle Rock Studios became a part of Valve and they were renamed internally
Valve South. With the acquisition Valve (Seattle) finally hopped on the game themselves, and gave it a
huge graphical/art/polish makeover. So in the last ~10 months or so of production the game was co-developed by both Valve South and Valve.
After production finished, there was the realization that due to the studio being far away (California) from Valve HQ (Seattle) the developers couldn't really work the way Valve employees like to do; move around, pick their favorite idea and make a project out of it (total freedom). So they asked those interested to move to Valve's Seattle studios.
9-11 Valve South employees moved to Valve and the rest of the company was small enough that Valve shut them down. Michael Booth was one of the people that did not move to Valve.
Valve began development of L4D2, now handled internally.
Booth and the rest of the gang reformed, and named themselves Turtle Rock Studios again. Turtle Rock Studios once again did contract work for Valve, co-developing DLCs for L4D1 & 2.
A more proper revival of the company was done in 2009?, 2010? by new CEOs, and at some point Michael Booth, the very creator of L4D, left. He now works for Blizzard making their new unannounced game.
In short:
L4D1: mostly developed by Turtle Rock Studios, but Valve gave it a big makeover in the final year of production. Valve also acted as producers all the way through the multi-year production.
L4D 1 & 2 DLCs: both
L4D2: Valve
L4D3: Valve (as far as we know
)
