I had implantation bleeding about 10DPO, it lasted only a day and was very faint pinkish spotting. KellyM is right, implantation bleeding usually occurs about 8-10DPO, sometimes up to 12DPO. I didn't have any cramping, I actually thought it was the beginning of my period, but 2 days later I got a BFP. It's highly unlikely you had implantation bleeding at 3-4DPO. If you had a positive OPK, usually you will ovulate within 24-36 hrs give or take. I wish you well!
Hmm, Implantation doesn't normally happen until 8-10dpo. I usually have it 2 days before af. Good luck to you!