When to test??

I know this is probably such a repetitive question, but what's the soonest anyone has ever tested after the DPO?? I am 4 DPO and I am TTC baby #2.. We had a miscarriage in November... And I got a normal period in December and tracked my ovulation date with the app and taking an ovulation test. I know it maybe too soon to TTC after a miscarriage but worth the try. What are earliest tries after ovulation for testing??