I received the following answer from "sabbahillel" as a comment, with the added note "Since I do not have a source for this, I am leaving it as a comment"
Since there would be no way of being sure that all four eggs have no
blood spots, returning 4 clean eggs would probably not be ribbis. One
is returning 4 eggs of the same status as those received. That is,
probably clean, but not certain.
In a lecture on the laws of Ribbis which I heard from R' Pinchos Vind Shlita of Yerushalayim, who is a renowned expert on ribbis issues, and author of many works on the subject of ribbis, he mentioned the scenario discussed here, and he ruled that there would be no problem of ribbis in returning 4 eggs. His reasoning for this ruling was exactly the reason given by sabbahillel in his comment, with slight elucidation.
Rabbi Vind said "since in todays world, when a person purchases eggs, and finds a bloodspot in one of them, it is accepted that he cannot demand the seller to replace the egg with another one. This shows that the price paid for eggs is for eggs which are "probably clean", and thus finding a bloodspot after the purchase does not grant the buyer any right to reimbursement. Based on this, if one borrowed 4 probably clean eggs, and returns 4 probably clean eggs, he is returning eggs of the same value which he borrowed, and no ribbis is involved. The fact that the borrower found a bloodspot in one of the eggs after borrowing the eggs does not change the value of the eggs at the time which they were borrowed."
[So, this is the source for Sabbahillel's comment, and thus it is hereby elevated to the status of an Answer.]