A Heckler & Koch Mark 23 is used by Mike Ehrmantraut (Jonathan Banks) again in "Bullet Points" (S4E04) to take out a pair of Ciudad Juárez cartel gunmen who ambush a refrigerated truck transporting methamphetamine.
In "Salud" (S4E10), Mike takes a Beretta 92FS Inox from one of Don Eladio's (Steven Bauer's) henchmen. His Beretta appears to be one of the later production model Inox versions (which has black controls).
In "Salud" (S4E10), after Mike tells him to "make [himself] useful" by getting himself a gun, Jesse Pinkman (Aaron Paul) grabs a Taurus PT99 AFS from a dead henchman. He later uses it while escaping Don Eladio's compound.
Walter White purchases a Ruger LCR from an illegal arms dealer (Jim Beaver) along with a leather I.W.B. holster in "Thirty-Eight Snub" (S4E02). The arms dealer recommends he load it with 158 grain hollow points for good stopping power, and also recommends he not cross draw unless he is sitting down. Although the Ruger is seen quite a bit in season 4, Walt only successfully uses it once. Walt later retrieves one from the soda machine in the car wash in the episode Confessions, and uses it to search his house for Jesse.
In "Bullet Points" (S4E04), a refrigerated truck guarded by Mike (Jonathan Banks) is ambushed and then shot up by two Ciudad Juárez cartel enforcers, both of whom are armed with Heckler & Koch MP5Ks.
The M4A1 Carbine is seen in the hands of one of Gustavo Fring's (Giancarlo Esposito's) gunmen riding in the back of the refrigerated truck in "Cornered" (S4E06).
In "Cornered" (S4E06), the other of the two mercenaries guarding the refrigerated truck is armed with a Colt Model 933, the shorter-barreled version of the M4A1 carried by his accomplice.
In "Bug" (S4E09), Cartel enforcer Gaff uses a version of the Remington 700 bolt-action rifle with Badger ordnance BDM (Badger Detachable Magazine) bottom metal, and Barrett BORS equipped scope to kill one of Gus's henchmen, and fires several more shots at Gus.
