Diary of a Madman was a promotional magazine entry written for Cube Magazine in 2003. Written when "Hallucination" seemed to be the final iteration of Resident Evil 4, it is a summary of events leading up to the game, which the diary indicates as being set in January 1999. Page scans can be found at Unseen64.
Leon's diary is split up into various "Progress Reports". The first one, dated January 14, 1999, has Leon recollect his ordeal from September. He incorrectly describes William Birkin as being infected with the t-Virus. Leon knows that the US government has had some previous dealings with Umbrella, and describes his working for them as going from one evil organization to the next, but understands that in this situation he has to pick a side. Sherry has been kidnapped by an unknown third party who Leon suspects is not affiliated with the company (referencing the ending to the original Wesker's Report, which suggests Albert Wesker is keeping an eye on her). In Leon's investigation of Umbrella, he has traveled all the way to Central Europe, "where it all began".
In the "January 18" entry, Leon describes his questioning the locals, who describe the strange goings on in the area. He starts feeling an eerie presence, that there's something not right about the region; he also suspects many of the locals are Umbrella spies.
The following day, Leon explores yet another of Umbrella's "eerie mansions", but believes it has been abandoned. The power is out, and the area looks derelict. Relying only on a torch, Leon explores the facility, hearing strange noises from the dark. He eventually finds a lone zombie in the building; it has no bite marks or other signs of attack, so Leon concludes it was deliberately infected for some purpose. He is confused as to how just one B.O.W. got out of the hidden lab, and not a horde.
Over the next two pages, various scribblings by Leon reminding him what he needs to do are present. Alongside these are files he has gathered, such as the ID card of the facility's director, Shinji Mikami. Leon becomes concerned that the reason there is a lack of zombies is because there is something in the mansion that eats zombies.
In Leon's "January 20" entry, Leon confirms that this was the mansion that developed the "Mother Virus", which the Progenitor Virus is based on (though, in series lore, the two are the same, with the former being a mistranslation). After hearing a strange chain being pulled around, Leon encounters the Hook Man, and fights it for a full hour, expending 30 Handgun bullets. Further in the entry, Leon's writing skills worsen, and he struggles to write a coherent sentence about how the Infected can turn into pure fog. Believing himself to be infected, Leon writes that he needs to find Chris soon.
4 Months in 1 Day
"4 Months in 1 Day" is a timeline that extends from p. 030 to the next page. The first entry is simply a summary of Resident Evil 2 (identified as "30 SEPT '98"). In Mid-October, Sherry has been placed in government custody owing to her G infection; Leon agrees to join an "underground anti-Umbrella organization". By Mid-November, Leon has begun his investigation; he contacts Claire and finds she is making progress with her own investigation (referring to her infiltration on the Paris lab in CODE: Veronica). When Claire contacted Leon at the end of December asking for help, Leon notified Chris who had to abandon his own Umbrella investigation (referring to his Europe trip referenced in Resident Evil 2) to come save her. By January 1999, Leon has collected enough information from Umbrella to begin his investigation.
An Umbrella History
"An Umbrella History" is Leon's summary of the viral incidents based on the information he has obtained. Spreading from p. 032 to the next page, it is a summary of five games set in 1998 that had already been released. While Leon has met Chris and Jill, and knows about Jill's ordeal with the Nemesis (though incorrectly identifying the Nemesis as being infected with the "NE-T Virus", rather than having the NE-Alpha parasite in its body), he makes no mention whatsoever of Resident Evil Survivor, when he sent his friend Ark Thompson to investigate the company.
The "Monsters Everywhere" section starts on p. 034 of that issue, and continues on to the next page. It lists the various viruses known as of 2003. The G-Virus entry is split in two, separating the primitive version Lisa Trevor was infected with from William Birkin's completed sample, which allowed him to implant embryos into other people's bodies. The t-Veronica Virus entry is significantly flawed; the section title misidentifies it as "T-VIRUS", and shows a picture of the Nemesis-T Type to go with it. The t+G-Virus (referred to as TG-VIRUS) is described as having the powers of both, with the benefit of having an "electromagnetic shield".