Check with your city or county to see if they have specifications for commercial driveways and/or a Standard Plan for commercial driveways. One or the other will have the base and concrete thicknesses required for driveways used by commercial vehicles. IIRC, my city requires 6 inches of concrete. Here's a typical city specification I found on the Internet:
2003.3A Driveways. All driveway approaches shall be 6” minimum thickness of concrete. All driveway approaches which, in the opinion of the Director of Public Works or his representative, will receive sufficiently heavy truck use to qualify as a “commercial” driveway shall have ½” (#4) reinforcing bars on 18” centers, each way, embedded in the concrete 2” – 3” above the base. Driveway approach widths and dimensions shall conform to the Department of Public Works Specifications contained in the brochure, “Parking, Loading, Driveway and Access Standards” and Standard Plan #2641 or #3134 as applicable. The driveway apron shall be constructed in such a way that a minimum 6” water barrier is maintained between the gutter flowline and the front sidewalk line (or within 4 feet of the gutter flow line where no sidewalk exists) unless specifically exempted by the Engineering Inspector.
2003.3A Driveways. All driveway approaches shall be 6” minimum thickness of concrete. All driveway approaches which, in the opinion of the Director of Public Works or his representative, will receive sufficiently heavy truck use to qualify as a “commercial” driveway shall have ½” (#4) reinforcing bars on 18” centers, each way, embedded in the concrete 2” – 3” above the base. Driveway approach widths and dimensions shall conform to the Department of Public Works Specifications contained in the brochure, “Parking, Loading, Driveway and Access Standards” and Standard Plan #2641 or #3134 as applicable. The driveway apron shall be constructed in such a way that a minimum 6” water barrier is maintained between the gutter flowline and the front sidewalk line (or within 4 feet of the gutter flow line where no sidewalk exists) unless specifically exempted by the Engineering Inspector.