"Biohazard 3.5" or "Resident Evil 3.5" are informal terms used to describe the various prototype stages for Resident Evil 4. Work on the game began in 2000 following internal changes to an existing game of the same name, and it was officially announced in 2001. Development ran into recurring problems with both the need to be technologically ambitious and revitalise the series and, ultimately, the game released in 2005 was very different from earlier builds.
Internally, all three prototype builds and the release version are the same game in terms of assets, programming and crewmembers. Many ideas from 2000 continued in some way until the game was heavily redesigned in early 2004, with the three prototypes being nicknamed "Castle", "Hallucination" and "Zombie".
Version 1: Castle
- Main article: Biohazard 4 (Castle Version)
"Castle" is the prototype most true to the original pitch, itself being a re-working of the "Stylish" build of the previous Resident Evil 4 project. The project was led by Yasuhisa Kawamura and Hiroshi Shibata, based on a pitch that would see Leon invade Umbrella HQ. The story was tackled by Noboru Sugimura and Yasuhisa Kawamura, and would have revealed that Progenitor Virus was isolated within the mummified remains of a superhuman who ruled over a vast kingdom in ancient times. as well as a plan by Oswell E. Spencer to gain immortality through it. In the game's opening, USSTRATCOM was to attack Umbrella HQ at the same time as Albert Wesker's HCF, leading to both factions being wiped out with the exception of Leon, who would be infected with a retrovirus and slowly mutate over the course of the game. Along the way, Leon was to meet up with a mysterious woman who was imprisoned with its lab, and is protected by a trained B.O.W. dog.
Technology-wise, the game was intended to use the GameCube's full resource capabilities, with each area being rendered fully in 3D like Resident Evil CODE:Veronica rather than using pre-rendered backgrounds. One of the important enemies of the game was "Black Fog", a floating mass of tentacles which was to infect Leon. Capcom was unable to develop realistic and resource-light animations for the enemy, forcing Shibata to oversee changes in how the game would appear. The only footage of "Castle" available is the TGS 2002 trailer. The "Castle" script was later handed over to Studio 2 for Haunting Ground, which kept its focus on the girl, her dog and her relationship with the villain with the complete removal of Leon.
Version 2: Hallucination
- Main article: Biohazard 4 (Hallucination Version)
The "Hallucination" build is an evolution of "Castle" which offered a radical change in how the game would be experienced, with Kawamura pitching a more psychological game with hallucinatory enemies instead of mutants. The story itself was the same as that of "Castle", though the script was to be re-written to better establish the change in situation. The only footage of this build was released at TGS 2003.
This version of the game based based around a complicated mechanic where two versions of a room would load each time, one in the real world and one in the hallucination world containing the enemies. By choosing this mechanic, the player would instantly cross between worlds when crossing a checkpoint without having an additional loading sequence. This again would prove the game's undoing, as the GameCube was not powerful enough to load all necessary assets. With the experiment being deemed a failure, Kawamura left the team.
Version 3: Zombie
- Main article: Biohazard 4 (Zombie version)
The third build of the game, "Zombie", saw the removal of the hallucination mechanics. The story was kept the same as with "Castle", with the main difference being that the main enemy would be Zombie-like creatures called Dabamen, which would be controlled by parasitic organisms. This build began work at some point after TGS 2003, and was quickly halted by executive producer Shinji Mikami following concerns by the company that the game's sales would stagnate if it were too formulaeic. No designs are known from this version, and it is possible no physical work was accomplished before the idea was thrown out.
- Rely on Horror interview: Hiroyuki Kobayashi
- Kevin Gifford, Mark MacDonald (2005-04). "Afterthoughts: Resident Evil 4". Electronic Gaming Monthly (Ziff Davis Media Inc.) (190): 51–52.