I live in NE Ohio... I noticed after winterizing and closing our pool that the water level had dropped significantly in 1.5 months and the ground was wet around one side of the pool.

I went to the pool store and they recommended I open and attempt to find the leak before it freezes.

I did this today and found that the pool was leaking because the liner had a number of small holes at the site where the walls screw together.

Are the installers supposed to cover the screw heads with some type of material to keep them from puncturing the pool liner?

I used green food coloring and the color "stuck" all up and down that seam, so I used an underwater repair kit over the entire screw line... I am wondering if I should go back to the company and ask them to replace the liner... it's only 4 years old.

I think you might want to ask for a new Liner. It never hurts to ask.