Well, Solidworks or Inventor will make an easier job of it but you can certainly do it in AutoCAD.
If you extrude your tread profile (tread becomes solid), you can use a copy of your tire to subtract the correct radius on the tread. Then move the tread 4mm below the surface, polar array and subtract.
Now your tread depth will remain 4mm through out the tire face...
Also, best to use Layers and union the array into a single solid to make it a little easier during the subtraction.