While the G.3 and G.4 had been credited largely to Gaston Caudron, the R.4 was mostly the work of Rene Caudron. Appearing in prototype form in June 1915, the R.4 had a full-length fuselage and single fin and rudder. The unequal-span wings had three bays on each side, with ailerons on the upper wing only. As well as the twin-wheel main landing gear units and tailskid, there was a single nose-wheel intended to protect the propellers. Power was from twin 97kW Renault i2Db engines. The three-man crew included nose and midships gunners each provided with twin Lewis machine-guns.
Intended originally as a bomber, it served mainly as an A.3 category three-seat reconnaissance aircraft, frequently engaged in photographic work. Its climb rate was not impressive, and a few aircraft were built with more-powerful 112kW Hispano-Suiza 8Aa engines in an attempt at improvement.
The production aircraft began to reveal a structural weakness. Among the crashes that happened, on 12 December 1915, a series aircraft under test was destroyed and Gaston Caudron, who was piloting the aircraft, was killed.
In early use Escadrille C.46 had claimed 34 German aircraft brought down with its R.4s in an eight-week period, but it was soon clear that in addition to structural redesign, improved ceiling and greater manoeuvrability were highly desirable. The new Caudron chief designer, Paul Deville, designed a new improved development which was to emerge as the R.11.
Production of the R.4 was terminated after 249 had been built. In the reconnaissance escadrilles it was replaced by the more-powerful Letord 1 during 1917.

Engine: 2 x 97kW Renault 12Db inline piston engines
Take-off weight: 2330 kg / 5137 lb
Empty weight: 1710 kg / 3770 lb
Wingspan: 21.1 m / 69 ft 3 in
Length: 11.8 m / 38 ft 9 in
Wing area: 70 sq.m / 753.47 sq ft
Max. Speed: 136 km/h / 85 mph
Ceiling: 4600 m / 15100 ft
Armament: 4 x 7.7mm machine guns, 100kg of bombs

The limitations of the G.3 and its inability to lift a worthwhile bomb load led to the development of the twin-engined Caudron G.IV as a dedicated bomber, which first appeared in March 1915. Structurally similar to the G.3, the G.4 had increased wing span, a tail-plane with four rudders instead of two, and twin engines mounted on struts between the wings. Power was provided by either 80 hp / 60kW Le Rhone rotary engines with circular cowlings, or by uncowled 75kW Anzani radials. The short crew nacelle had an observer/gunner’s cockpit in the nose, though the field of fire was limited by the proximity of the engines, and a bomb load carried underwing. Defensive armament was limited to usually the one 7.7mm machine gun and the bombload capacity varied between 220lbs and 250lbs. As well as a 7.7mm Lewis or Vickers machine-gun on a ring mounting in the front cockpit, some G.4s had a second Lewis gun mounted over the upper wing centre section for rear defence. This second gun could only be operated by the gunner standing at full stretch in his cockpit.

The G.4 was ordered into large-scale production and two versions were built, the Cau 4 B.2 day bomber and the Cau 4 A.2 artillery observation and reconnaissance aircraft. A number of the G.4 A.2 version had wireless installed.
The appearance of the G.4 was delayed by production problems and first appeared in service with the French Aviation Militaire in November 1915 as the Cau 4B.2. This particular model, however, suffered from a high loss rate and would be phased from service by November of 1916. The Cau 4A.2 followed and was put to more successful use in the reconnaissance and artillery spotting role.
The British Royal Naval Air Service purchased 55 of the type, 43 imported and 12 were built by the British Caudron Company. Flown by Nos 4 and 5 Wings, they were used in 1916 and early 1917 in attacks on German seaplane and airship bases in Belgium. The RNAS G.4s were replaced by Handley-Page O/100s in the autumn of 1917.
The Italian Aeronautica Militare received imported G.4s and was also supplied with 51 examples built by the A.E.R. company at its factory near Turin. In May 1917 the 48a Squadriglia was the first unit to re-equip with the G.4, to be followed by the 49a and 50a Squadriglie. Italian G.4s operated in the mountainous Alpine areas, demonstrating their good climb qualities and suitability for flying at altitude.
During the war the type established several Italian altitude records. A number of Caudron G.4s were also supplied to the Imperial Russian Air Service, with which they flew in the reconnaissance role with onboard radio.

Towards the end of World War I G.4s were relegated to training duties, and 10 of the type were bought for that purpose by the United States Air Service in France in early 1918. The G.4 was used for a number of notable post-war flights and some hundreds were sold to private owners and flying clubs in France and Italy post-war.
In France 1,358 of the G.4 were produced of the 1,421 G.IV built in all.

Caudron G.4A.2
Engines: 2 x Le Rhone 9C rotary, 80hp
Length: 23 ft 8 in (7.16m)
Wingspan: 56 ft 5 in (17.20m)
Wing area: 36.8 sq.m / 396.11 sq ft
Height: 8 ft 6 in (2.60m)
Maximum Speed: 82mph (132kmh; 71kts)
Service Ceiling: 14,108ft (4,300m; 2.7miles)
Armament: 1 or 2 x 7.7mm machine guns, 220lbs of external ordnance
Accommodation: 2
Hardpoints: 2
Empty weight: 733 kg (1,616 lb)
Maximum Take-Off Weight: 1,232 kg (2,716 lb)

Caudron G.4A.2
Engines: 2 x Anzani 10-cylinder radial, 100hp
Length: 23 ft 8 in (7.16m)
Wingspan: 56 ft 5 in (17.20m)
Wing area: 36.8 sq.m / 396.11 sq ft
Height: 8 ft 6 in (2.60m)
Maximum Speed: 82mph (132kmh; 71kts)
Service Ceiling: 14,108ft (4,300m; 2.7miles)
Armament: 1 or 2 x 7.7mm machine guns, 249lbs of external ordnance
Accommodation: 2
Hardpoints: 2
Maximum Take-Off Weight: 2,932lbs (1,330kg)