Messerschmitt Bf 109K-4.
[Source: Unknown]

The wings featured the large rectangular fairings for the large 660 × 190 mm (26 × 7 in) main wheels. Small wheel well doors, originally planned for the G series, were fitted to the outer ends of the wheel bays, covering the outer wheels when retracted. These doors were often removed by front-line units. The radio equipment was the FuG 16ZY with an antenna mast fitted under the port outer wing and FuG 25a IFF as well as the FuG 125 Hermine D/F equipment. Internally, the oxygen bottles were relocated from the rear fuselage to the right wing. Flettner tabs for the ailerons were also to be fitted to serial production aircraft to reduce control forces, but were extremely rare, with the majority of the K-4s using the same aileron system as the G series.

Armament of the K-4 consisted of a 30 mm MK 108 engine-mounted cannon with 65 rounds, and two 13 mm MG 131s in the nose with 300 rpg, although some K-4s were fitted with the MG 151/20 in place of the Mk 108. Additional Rüstsätze, such as a 300 L (80 US gal) drop tank (R III), bombs up to the size of 500 kg/1,100 lb (R I), underwing 20 mm Mauser MG 151/20 cannon gondola pods (R IV) or 21 cm (8 in) Wfr.Gr. 21 rockets (as on the Gustav models) could be carried after minimal preparations; the latter two however were rarely used by Bf 109 units at this stage of the war, although III./JG 26 were almost completely equipped with K-4s which were fitted with R IV.

In addition there were problems with the 30 mm MK 108, which often jammed while the aircraft was manouevring in battle, leaving the pilot to fight on with the two heavy machine guns. The standard Revi 16C reflector sight was fitted, which was slated to be replaced later by the EZ 42 Gyro gunsight, although this never happened.

Power was provided in production K-4s by a Daimler-Benz DB 605DB/DC engine (very early K-4s used the earlier DM).The DB/DC engine had an adjusting screw allowing the engine to use either B4 + MW 50 Methanol Water injection equipment or C3 fuel (DB 605 DB) or C3 fuel, with or without MW 50 (DB 605 DC). Using B4 fuel with MW 50, the DB generated an emergency power rating of 1,600 PS at 6,000 m (1,160 PS maximum continual at 6,600 m), and take-off power of 1,850 PS at 0 m, with a maximum supercharger boost of 1.8 ata. The DB could also be run on higher octane C3 fuel, but use of MW 50 was forbidden. The DC ran on C3 fuel and could generate a potential 2,000 PS, but only when using C3 fuel with MW 50 and a boost of 1.98 ata, otherwise the power ratings were similar to that of the DB. A wide-chord, three-bladed VDM 9-12159A propeller of 3 m diameter was used, as on the G-6/AS, G-14/AS and G-10.

Deliveries began in mid-October 1944. 534 examples had been delivered by the Messerschmitt A.G., Regensburg by the end of November 1944, and 856 by the end of the year. Regensburg delivered a total of 1593 by the end of March 1945, after which production figures are missing. With such a high rate of production, despite continuous heavy fighting, by the end of January 1945 314 K-4s – about every fourth Bf 109 – were listed on hand with the first line Luftwaffe units. Ultimately it was intended to equip all Bf 109 units with the 109K, which marked the final stage of 109 development before the jet age.

Using MW 50 and maximum boost the Bf 109 K-4 was the fastest Bf 109 of World War II, reaching a maximum speed of 710 km/h (440 mph) at 7,500 m (24,610 ft) altitude. Without MW 50 and using 1.80 ata the K-4 reached 670 km/h (416 mph) at 9,000 m (26,528 ft). The Initial Rate of climb was 850 m (2,790 ft)/min, without MW 50, and 1,080 m (3,540 ft)/min, using MW 50.

The Bf 109 remained comparable to opposing fighters until the end of the war. However, the deteriorating ability of the thousands of novice Luftwaffe pilots by this stage of the war meant the Bf 109's strengths were of little value against the numerous and well-trained Allied fighter pilots.

Additional development:

Several other versions were projected based on the 109K airframe – K-6, K-8, K-10 and K-14. In the proposed K-6 the armament would have been two 13 mm MG 131 above the engine, along with a 30 mm MK 108 and an internally mounted MK 108 in each wing, with 45 rpg. Alternatively, the wing MK 108s could be substituted by 20 mm MG 151/20s, with 100 rpg. Armour weight was increased to 90 kg (200 lb). Takeoff weight was 3,600 kg (7,900 lb). Some K-6 prototypes were built and tested at the Erprobungstelle Tarnewitz weapons-testing centre on the Baltic coast.

Project drawings of the K-8 show a K-series airframe powered by the two-stage DB 605L high altitude engine, a high-velocity 30 mm MK 103, and two 30 mm (1.18 in) MK 108 cannons in the wings; the cowl 13 mm MG 131s were dispensed with.

Some sources point to limited use of the K-14, intended as high-altitude heavy fighter. Two airframes are listed as delivered to II./JG52 under Major Wilhelm Batz in late spring of 1945, these being armed with only one 30 mm cannon, but the type's existence cannot be positively confirmed. The K-14 was to be powered by the two-stage supercharged DB 605L engine, using a four-bladed propeller. 760 km/h (470 mph), and an operational altitude of 12,000 m (39,000 ft) was projected. Armour and armament were otherwise similar to the K-6.