Mayhem In Metropolis (Pages 4-5)
Page 4, Panels 4 and 5 contain a couple of familiar cameos. Lex Luthor was created by Jerry Siegel and Joe Shuster and first appeared in April 1940's Action Comics #23. George Pérez designed his now-familiar chunky green battlesuit, which first appeared in June 1983's Action #544 (incidentally, the 45th anniversary of Superman's first appearance). In fact, for over a year Luthor has worn a blue Superman-inspired battlesuit, complete with red cape; so this is something of a return to form. Brainiac was created by Otto Binder and Al Plastino and first appeared in July 1958's Action #242. He too was redesigned for Action #544, albeit by Ed Hannigan. In Alan Moore and Curt Swan's seminal "Whatever Happened To The Man Of Tomorrow?" (September 1986's Superman #423 and Action #583), the hexagon-adorned Brainiac skull attached itself forcibly atop Luthor's head, and it looks like something similar (and perhaps more consensual) is going on here with Superman.
We're not sure if the third person in Panel 4 is Jimmy Olsen or the Joker, or some macabre combination of both. Since we'll see Jimmy (sort of) on Page 5, we'll note that he was also created by Siegel and Shuster. Following an anonymous cameo in November 1938's Action #6, his first named appearance was on an April 15, 1940 episode of the "Adventures of Superman" radio serial. He then had a proper introduction in November/December 1941's Superman #13. Jimmy being turned into a Doomsday creature is perhaps one of the character's darker transformations; and in a career which has seen him become a werewolf, a Bizarro, a giant-headed "future man," and a towering Turtle Boy monster, that's saying something. In fact, it looks like Doomsday-Jimmy has little red horns where his red hair and freckles would be.
Mister Miracle's Metal debut came in the Forge one-shot, so we'll just note that the helmeted fellow on Page 4, Panel 5 is Orion the Hunter, who was created by Jack Kirby and appeared first in February/March 1971's New Gods issue #1. Orion is also appearing presently both in the Mister Miracle miniseries and in Hal Jordan and the Green Lantern Corps.
Page 5 notes that it's been seven days since the end of Metal issue #2. As it happens, the "Gotham Resistance" crossover tie-in takes place two days after the appearance of Challengers Mountain in Metal issue #1. Wonder Woman #32 also refers to the events of Metal #2, so since we saw Wonder Woman and Superman trapped at the end of that issue, spoilers -- looks like she might make it out of Metal alive.
While we'll probably get the lowdown on the Doomsday-fied Batman called "Devastator" in a future one-shot, the Doomsday Virus (to which all of Metropolis seems to be succumbing) first appeared in May 2014's Superman: Doomed issue #1 (written by Scott Lobdell, Greg Pak and Charles Soule and drawn by Ken Lashley).