1) Why is 1knt of current = 4knts of wind ?
Water is denser that air.
I was taught a rule of thumb tHat water is 15 times denser than air- have no idea if this is accurate, but it helps me to fathom current vs. Breeze issues.
2) Laying a mark: sailing near shore the wind will have big shifts ( lifts and headers ) . tide will also change quite a bit depending on bottom, there might also be reverse eddies. Therefore, one needs to be well aware of local conditions up the course and be aware that the conditions at the make may be different than 1 mile away.
This is what make sailing such a pleasure - the playing field is a constantly changing surface