How much will it cost me to change a 6' box to a 4' box on a truck?
Do you mean 6ft to 8ft?
thousands. you would have to buy a new bed, and modify the frame, all wiring, driveshaft, possibly fuel system. if it's already a custom truck it might be worth it, otherwise either deal with it or sell it and buy a truck with the bed size you want. and your insurance company may have issues with that level of modification as well.
Search Ford F-150 Questions
Ford F-150 Experts