Altogether there is 6 Citizens E's.
Here is a video showing the locations of the 4 missable Citizen E's (it is best to go through the storyline then find the 4 in free roam).
The remaining 2 of the citizens are apart of the main story and cannot be missed. (Hence why they are not included in the video guide.)
One is in Sequence 2, Memory 6 and the other one is at the very end of the story after seeing the alternative ending.
I do not own this video nor did I make it. All ownership goes to PowerPyx on YouTube.