I would tend to think it was a cyst and not a pregnancy, because a pregnancy that early would not be painful to lose. Ovarian cysts, on the other hand, can hurt like crazy. If you have been trying to conceive, your ovary can get bonked at the time of ovulation from the sex, and can bruise, and it can puff up and be more likely to get a cyst that doesn't drain. Those do hurt.
I also think that even 4 days after the bleeding began, there would probably be some trace hCG in your system if this kind of pain was from an embryo miscarrying.
Its possible you had a chemical pregnancy. That's a super early miscarriage. Another possibility is that you had a cyst and are passing it. That is what happened to me when I had symptoms like yours. You will likely never know so just move ahead for the next cycle of trying. Good luck with it al!