I saw someone else explain it elsewhere on the boards that it depends on your baseline HCG, which can apparently vary from 1-5. So, if it doubles every day, and you start at 5, then a test that is sensitive to 10 will pick you up the day after you implant. So they would be 8DPO. But if you start at one, you need to go 1-2-4-8-16 - five days before you can be picked up on a test! But I doubt you were implanting at 4DPO. Between 6-8DPO is the 'norm' for implanting and sounds about right for you if your baseline HCG is ~2.