In the first dream sequence back in season 1, Elliot is proposing and wearing marriage clothes with Angela. Now, in season 4 we just found out, that in an alternate reality Elliot is, in fact, marrying Angela. Also, Elliot is an only son (Darlene doesn't exist) and his dad is actually a pretty cool guy that is his friend. At the end of the episode, we see our original Elliot inside this new alternate reality. Our original Elliot comes to this alt. reality around the same time that the first season takes place.
My theory is that our original Elliot gets to decide if he wants to stay in this alternate reality, and to do so, he will have to replace the Alternate Elliot. Somehow, he manages to send Alt. Elliot to the "original" reality, back to the beginning of the show. This alt. Elliot is the person we actually see through the show and since Alt. Elliot doesn't have a sister, of course, he "forgot" who Darlene was, of course, he thought his dad was his best friend because those were actually memories from another dimension.
This is the Alderson loop, Elliot replacing Elliot, replacing Elliot, replacing Elliot. This might be the 6th iteration since this time around it is called F corp, so maybe there was an A corp, B corp, and so on.
Also, I might be wrong but these alternate reality scenes are taking place on May 9th, meaning 5/9... This means we might finally found out where Elliot was during those 3 days