M.G.
ETA:
Changed my answer. I think you'd benefit from talking to a child therapist along with your daughter, or to the school counselor.
We had a little girl who was 'teased' at our school around that age. From there, she would purposefully do things that were considered a bit 'gross' just to get a reaction from the boys when taunted. Or she'd run after them. Your description reminds me of it a bit. I'm no expert, but looking back, I think negative attention was attention. She didn't have a lot of friends.
A lot of kids get teased and the "So and so likes you".
Your daughter's reaction unfortunately is now provoking it. And now, as you say, she's obsessed. She also doesn't know the meaning of respecting boundaries - for herself or this boy. That's where therapy might be helpful at this point. I agree, it's too bad no one stepped in but it's not too late to just learn from this.
I think nip it in the bud over summer and get some help for her before fifth grade. Talking to the counselor at school will also help with next year's teacher. I like 2kidmama's suggestions for activities, etc. Good luck :)