4" or so? More important is the comfort of the height of the seat to the countertop. Seat too low causes the elbows to be too high.

Sit at a comfortable table/desk and measure seat to top distance. Use that for shortening the stools.

To shorten the stools, do not measure from the ends of the legs up. Too easy to make a mistake. Determine the new leg length and lake a story stick of that length. Be sure to cut the correct angles on each end.

Then, mark the two outside surfaces of each leg---keeping the correct positioning of the story stick.

Then, mark the inside surfaces. All the four lines on each leg should meet at the correct angle to now cut off the amount necessary. Be sure to allow for casters/etc.

I can answer this by saying the average chair seat height is 18" and the average table top height is 30" +-1" is generally accepted. So the distance from the seat to the top surface should be about 12" Anything shorter than 6" from the top of the seat to the bottom of the apron is generally considered too little thigh room.

I measure up from the bottom of each leg while the chair is sitting on a known flat reference surface.