WELCOME TO THE INSANITY!!!!!
It is only going to get worse from here on out.
Many great and old finds do occur at 4 inches or less. As you use you machine more, the depth from which you find things will probably increase. The more time you spend with your current machine, the better you will get with it. Stay with it, listen to what it tells you and take notes.
One of the other major factors is location, location, and location. The more research that you do, the more likely you are to find areas that were heavily used 60 or more years ago and look like nothing today. Like most people will agree, no place is ever hunted out. The more trash you remove from the ground at the shallow depths will let you find the targets that are just a little deeper.
I am sure that others will chime in and help you on your journey.
Approximately where are you located in the US? Some areas have "hot" soil and high minerals that could be working against you.
Good Luck!!
Bob