DC Barry Goddard (Jolyon Baker) holds a Smith & Wesson Victory Model revolver with 4" barrel in the climactic scene in "The Last Interview" (S04E01).
Unidentified revolver
Gang boss Joe Conte (Dennis Creaghan) carries a revolver in shoulder holster in "The Last Interview" (S04E01). When he draws the gun, it is seen very briefly and blurry.
Pistols
M1911A1
Gangster Dino Percotti (Michael John Paliotti) carries an M1911A1 pistol in "The Last Interview" (S04E01). In the climactic scene in "Return of the Ice Maiden" (S04E05) Liddell's Inglis Hi-Power (see below) switches to an M1911A1 when it falls in hands of Philippa Vale (Liza Goddard), due to a continuity error. Ahmed El-Mazri's bodyguards carry M1911A1 pistols in "Avenge O Lord" (S04E09).
SACM M1935A
Gangster Vince Marchetta (Keith Edwards) carries a SACM M1935A pistol in "The Last Interview" (S04E01).
Inglis Hi-Power
Liddell (Philip Hynd) carries an Inglis Hi-Power in "Return of the Ice Maiden" (S04E05). Due to a continuity error, when the gun falls in hands of Philippa Vale it switches to an M1911A1. The perfect close-up of the pistol reveals even the serial number starting with 0T that indicates the pistol was manufactured in July-August 1944.
Walther PPK
Haskins (Ian Sharp) holds a Walther PPK in "The Tennis Racket" (S04E07). When he is disarmed, DC Barry Goddard (Jolyon Baker) picks up the pistol and hands it to DS Jim Bergerac (John Nettles). The DVD cover for Season 4 that shows the titular character holding a PPK probably refers to this particular episode. In the final scene in "Sins of the Fathers" (S04E08) Henry Hoffman (Warren Clarke) is briefly seen with a pistol that seems to be a PPK.
Walther P38
Henry Hoffman (Warren Clarke) and several extras in the roles of German officers carry Walther P38 pistols during the set of a film about Jersey's occupation by the Nazis in "Sins of the Fathers" (S04E08).
Reck Baby
In one scene in "Sins of the Fathers" (S04E08) a local man (Roger Ostime) whose father was executed by Nazis shoots a blank firing Reck Baby pistol at Henry Hoffman when the latter appears in the role of an officer in charge of the firing squad. DCI Barney Crozier identifies the pistol as a starter gun.
Unidentified pistol
A pistol is seen on the table in Alfred Vaccarro's (Maurice Roëves) hotel room in "Sins of the Fathers" (S04E08). This gun may be an M1911A1 but it's a vague guess.
Submachine Guns
MP40
MP40 submachine guns are seen during the set of a film about Jersey's occupation by the Nazis in "Sins of the Fathers" (S04E08).
MAC-10
Mick Davies (Ian Redford) is armed with a MAC-10 submachine gun in "Avenge O Lord" (S04E09). He claims that "this machine pistol sprews out 20 rounds a second" that matches the 9x19mm version (1200-1500 rpm vs 1050 rpm on the .45 version).
In the climactic scene in "The Last Interview" (S04E01) police snipers are armed with bolt action sniper rifles that seem to be Weatherby Mark V.
Takedown M1 Carbine
The hitman John Causebrook (Bill Weston) uses a takedown M1 Carbine with sniper scope and sound suppressor in "Low Profile" (S04E04). This is exactly the same prop that is seen in Season 2.
Karabiner 98k
Karabiner 98k rifles are seen during the set of a film about Jersey's occupation by the Nazis in "Sins of the Fathers" (S04E08). Mostly these are late productuion rifles with hooded front sights and simplified barrel bands.
Mauser Gewehr 1898 Carbine
At least one Mauser carbine seen in "Sins of the Fathers" (S04E08) has Lange-Visier rear sight. Probably this rifle is a Gewehr 98, shortened to carbine length. Similar rifles are seen in a number of British war movies.
Ruger Mini-14
A police sniper (Lorcan Cranitch) is armed with a Ruger Mini-14 with sniper scope in "Avenge O Lord" (S04E09). The gun is fitted with a kind of aftermarket foregrip.
Assault Rifles
Adler-Jäger AP-80
In the flashback scene in "The Last Interview" (S04E01), depicting journalist Howard Bailey's mission in Africa, numerous soldiers of unnamed African state are armed with AK-47 style assault rifles. Judging by the slim and less curved magazines, these guns are not genuine AKs but Adler-Jäger AP-80 replicas, chambered in .22 caliber. These guns are fairly common for 1980s British productions.
Machine Guns
MG34
MG34 machine guns are seen during the set of a film about Jersey's occupation by the Nazis in "Sins of the Fathers" (S04E08).
Trivia
A Royal Navy Westland Sea King HAR.5 search and rescue helicopter is seen in the final scene in "Sins of the Fathers" (S04E08).