Soil test says to go 4in but when i go that deep below my grass its all clay should i test that deep then?

i have a capsule test with little test tubes. it tells me to dig down 4 inches and test that soil but that deep all it is clay. should i test the clay or only go a inch down to were the grass soil is ? also should i test my lawn for potash nitrogen and phosphorus ? i did pH so far and it came out fine but i only tested the soil the grass thrives in that's 1 inch deep.