Implantation occurs after 9-12 days from conception. You will get a positive pregnancy test after a week from missed period or later.

It isn’t merely about drinking water and eating. Your urine must be concentrated. If you did go to the loo since morning, it’s less effective to test in afternoon. Unless you’re far ahead in pregnancy you’ll have false results. As your results are positive despite taking it late most likely they are true and sign that you’re pregnant weeks more than you’re expecting. Looking at an old pregnancy test is of no use because it is no longer valid after few minutes. The line you saw after four days on the old used test was evaporation line most probably.