I dont think that a lot of SE3 players has abandoned the series after SE4 release. The most players had complaints about some good SE3 game mechanics which changed in SE4, but nonetheless played it like hell.
In SE4 I missed always the excellent construction system of SE3. I also dont like that fighters in SE4 are hardcoded to be able travelling in-system. (this should be rather an option for special long range fighters, while the standard fighter should be carrier or planet based) But these are just minor gripes.
I dont think that the differences from SE3 to SE4 are fundamental. But I am not so sure that there is no fundamental difference between SE4 and SE5. I cannot believe that it is easy for a normal user to construct 3D shipsets. Also I am VERY sceptical about the real time tactical combat engine. This is a big change to the classical SE approach to this matter. Many traditional turn based gamers will leave the fanbase because of this. (maybe me included) And because of a lot of newcomers which are attracted by realtime games the whole structure of the SE-fanbase will change.
For us all I hope that MM has learned something about the MOO3 desaster.
KlausD