E-3To improve the performance of the Bf 109E, the last two real prototypes, V16 and V17 were constructed. These received some structural improvements and more powerful armament. Both were the basis of the Bf 109 E-3 version. The E-3 was armed with the two MG 17s above the engine and one MG FF cannon in each wing. A total of 1,276 E-3 were built, including 83 E-3a export versions.
E-4The E-3 was replaced by the E-4 (with many airframes being upgraded to E-4 standards starting at the beginning of the Battle of Britain) which was different in some small details, most notably by using the modified 20 mm MG-FF/M wing cannon and having improved head armour for the pilot. With the MG FF/M it was possible to fire a new and improved type of explosive shell, called Minengeschoß (or 'mine-shell') which was made using drawn steel (the same way brass cartridges are made) instead of being cast as was the usual practice. This resulted in a shell with a thin but strong wall, which had a larger cavity in which to pack a much larger explosive charge than was otherwise possible. The new shell required modifications to the MG FF's mechanism due to the different recoil characteristics, hence the MG FF/M designation.
The cockpit canopy was also revised to an easier-to-produce, "squared-off" design, which also helped improve the pilot's field of view. This canopy, which was also retrofitted to many E-1s and E-3s, was largely unchanged until the introduction of a welded, heavy-framed canopy on the G series in the autumn of 1942. The E-4 would be the basis for all further Bf 109E developments. Some E-4 and later models received a further improved 1,175 PS (1,159 hp, 864 kW) DB601N high-altitude engine; known as the E-4/N; owing to priority being given to equipping Bf 110s with this engine, one fighter gruppe was converted to this version, starting in July 1940. The E-4 was also available as a fighter-bomber with equipment very similar to the previous E-1/B. It was known as E-4/B (DB 601Aa engine) and E-4/BN (DB 601N engine). A total of 561 of all E-4 versions were built, including 496 E-4s built as such: 250 E-4, 211 E-4/B, 15 E-4/N and 20 E-4/BN.