E-4 differred from the E-3 by having 'different' armament. 'Different' because it had MG FF/Ms capable of firing Mine shells, but the gun itself only differed in it`s recoil elements adjusted to the lighter shell.
Externally, I believe the E-4 also had a new type of canopy, a more boxy one, much like as on the 109Fs, whereas the E-3 had more rounded shaped canopy with some curved plexi surfaces.
I am others will turn up here and give a more satisfying answer on the matter.
