Throughout season 4 Det. Mac Taylor (Gary Sinise), Det. Don Flack (Eddie Cahill), Det. Jessica Angell (Emmanuelle Vaugier), Dr. Sheldon Hawkes (Hill Harper) and Det. Danny Messer (Carmine Giovinazzo) all carry Glock 17s. Also, any NYPD officer seen with a pistol is using a Glock 17 unless otherwise indicated on this page.
In "Can You Hear Me Now?" (S04E01), Charles Price (Erik Fellows) (one of the guards at the Statue of Liberty) is seen with a Glock 17 during a flashback.
In "Admissions" (S04E18), Inspector Stanton Gerrard (Carmen Argenziano) uses a Glock 17 to kill the man who raped his daughter.
Throughout season 4, Det. Stella Bonasera (Melina Kanakaredes) and Detective Lindsay Monroe (Anna Belknap) carry a Glock 26 as their sidearm. On at least one occasion Bonasera is seen with a Glock 19 instead.
In "Down the Rabbit Hole" (S04E05) and "DOA for a Day" (S04E15), an assassin known as "Suspect X" (Kam Heskin) carries a Colt Commander with chrome finish, pearl grips, and a suppressor as her main weapon of choice. She notably uses it to kill a Congressman and later shoots a hostage to facilitate her escape. In "DOA for a Day" notably when she gets into a gunfight with Mac in the climax of the episode, using it until she is wounded in the chest by a round from Mac's Glock 17.
In "Hostage" (S4E21), a bank robber named Ethan Scott/Douglas Anderson/'Joe' (Elias Koteas) can be seen armed with an M1911A1 pistol as his main weapon.
In "Buzzkill" (S04E08), a kidnapper is seen with a Smith & Wesson 6904 pistol. When Mac and Danny close in to arrest the kidnapper, the gun is seen on a nightstand.
In "The Thing About Heroes..." (S04E10), Drew Bedford (Kerr Smith) kidnaps Mac and puts him in a chair with a stainless Ruger GP100 pointed at him. Surrounded by lasers, if Mac moves, he will be shot. Mac manages to trigger the device so that it shoots Drew and then grabs the gun shooting him in the arm with it a second time.
In "Commuted Sentences" (S04E07), a woman armed with a Smith & Wesson Model 640 is killed when an attacker grabs her from behind, turning the gun on her and forcing her to be shot with her own weapon. The is found by the CSIs at the crime scene.
In "Commuted Sentences" (S04E07), Det. Danny Messer (Carmine Giovinazzo) is seen pulling his backup weapon from his ankle holster. He and Lindsay remove the bullets and use them, and a slingshot, to determine the trajectory of a bullet.
In "The Deep" (S04E02), ESU officers are seen with Heckler & Koch MP5A2s while detaining a terrorist attempting to blowup a dock.
In "Down the Rabbit Hole" (S04E05), an ESU officer is seen with a Heckler & Koch MP5A2 when searching for "Suspect X".
In "The Thing About Heroes..." (S04E10), ESU officers are seen with Heckler & Koch MP5A2s while searching for Drew Bedford.
In "Taxi" (S04E20), ESU members are again seen with the Heckler & Koch MP5A2 when detaining the taxicab killer.
In "Hostage" (S04E21) , a TDI Vector (refered to as the Kriss Super-V) is determined to be the murder weapon after a victim is found with two bullets through the same entry wound. This is due to the weapon's unique recoil reduction system. Det. Stella Bonasera (Melina Kanakaredes) is seen test firing the gun with DHS Agent Brett Dunbar (Adam Baldwin). One the men who supposedly kidnapped the bank robber is also seen with the gun.
In "The Deep" (S04E02), a U.S. Coast Guard patrol boat armed with a bow-mounted M240B machine gun is seen among Coast Guard and N.Y.P.D. air units and surface vessels sent to intercept a terrorist diver attempting to blow up a helipad on the bank of the Hudson river where a foreign dignitary is arriving.
In "All in the Family" (S04E13), a Steven's Arms Double Barrel Shotgun is identified as the gun used in two murders. It is first used to kill a Judge, then is thrown off a roof where it kills a woman after striking the side of the building and discharging. Lindsay is also seen test firing the gun.
