You're right it would be O-4 or O-3 depending on if ovulation was Thursday or Friday. I think when I originally read the post I was thinking ovulation was Friday and I personally wouldn't be worrying about a Sunday attempt. But since ovulation was probably Thursday it would be 2 attempts, as atomic said.
I think the easiest thing to do is just BD every 4 days, then you don't need to worry about it. :)