interestingly, this fugue was a remade version of a very old sketch (from around three years ago) that i had given up on because my skills were then obviously very lacking. i've just dug up the original version of this piece from which this fugue is based.
another detail i think is worth pointing out about measure 4 is that i had intended this fugue to go to five voices right from the conception of the subject, and when rewriting the piece, i had evaluated that initial decision to be fundamentally correct. however, i also wanted to avoid redundant subject entries (as for this many voices, the piece would feel very slow with three sets of subject-answer in the same two keys). therefore, there are two choices for me to consider: either the part of the exposition after the entry of the fifth voice leading up to the cadence is in the tonic (or mediant), or the entry of the fourth voice is in a different key (preferably the subdominant) to strengthen the arrival of the fifth and final voice. you can see both of these mechanisms at play in bach's BWV 849 (c# minor) and BWV 562 (c minor, incomplete but the entire exposition is there).
i recommend taking a look at the prelude as well, since it's considerably shorter than the fugue.