If I have implantation spotting on what I thought was 4 DPO (could also be 5 DPO). When is the soonest that I have a possibility for a BFP?

Our anniversary is coming up and I’d love to surprise him for that if this month is my month

EDIT: for clarification— my husband and I aren’t full force trying with ovulation tests etc, SO im not 100% on dates... but I was curious and got a positive LH test after BD. The test and cramping suggests I’m at 5 DPO, but glow says I’m 4DPO. I never have spotting except when my period is about to come (which isn’t the case).

I’m just curious, I have cheap tests that say they measure as low as 25 hcg. I’m not gonna be mad at the like BFN, I just don’t want to test EXTRA unnecessarily.