That 4-hole US Army button is made of zinc, and it dates from World War One. Here's a photo showing one, and its smaller mate, on a US Army uniform's pants whose manufacturer-label is dated March 15, 1918.
These zinc WW1 pants-buttons came in two sizes. Yours is the larger size, and says "US Army." The smaller version was for the pants fly, and says simply "USA."
Regards,
Pete