The hook effect is a phenomenon (supposedly its rare) where the concentration of the pregnancy hormone in your urine is so high that the test cant actually pick it up (i guess it just confuses it) I am 9 1/2 weeks pregnant, I got normal progression in early pregnancy, but took a test yesterday that was light (when it should technically be SUPER dark)
The reason theres 4 tests is that apparently if you suspect your tests are light because your hormones are super high you should dilute your urine with water, therefore diluting the hormone to a level the tests can pick up appropriately. So I took one with 100% urine, one with 50% water and 50% urine- gets a little darker, 75% water 25% urine, then finally mostly water with just a tiny bit of urine in it- and this was my darkest positive
Hope that helps!