I think the problem is that the first three coats were not cured. If you put those coats on in rapid succession, you would have a thick layer of paint with a dry skin, but still soft underneath. When you sprayed one coat the next day, it was thin and dried quickly. As it dried, it shrank, and moved the paint underneath, causing those ridges. The good news is you don't have to strip it. If you wait until all the paint is fully cured, you can sand down the ridges, and add one more coat to cover. Edit: After looking at the photo again, you may have to strip it. It looks like there are a couple of blisters. Those may have pulled the paint up all the way down to the primer. If you can wait until the paint cures, it's still worth a try sanding it. If you don't want to wait, strip it.