I'll take the risk of sharing my opinion. I think a 3–4-year-old child still can’t fully grasp the concept of “guilt.” That’s why they don’t understand what they’re supposed to apologize for. And I don’t think punishing them or making them stand in the corner until they say the “right” apology is a good practice. I believe it’s better to explain more, rather than force them to say “what you want to hear”
I had exactly the same situation with Oleg.