The Messerschmitt Bf 109 was a German World War II fighter aircraft designed by Willy Messerschmitt in the early 1930s. It was one of the first true modern fighters of the era, including such features as an all-metal monocoque construction, a closed canopy, and retractable landing gear. Having gone through its baptism of fire in the Spanish Civil War, the Bf 109 was still in service at the dawn of the jet age at the end of World War II, during which it was the backbone of the German Luftwaffe fighter force.

The E-4 saw the engine mounted cannon finally abandoned. The wing mounted cannons were upgraded to the MG FF/M, which gave a higher rate of fire. The cockpit canopy was also modified. It first appeared in July 1940, and played a significant role in the Battle of Britain. German figures for fighter losses show 249 E-1s, 32 E-3s and 344 E-4s lost in the second half of 1940. It would have been relatively easy to upgrade a 109E-3 to the E-4 specifications, and this is what may have happened to many of the older fighter.