4mm might do at a push but i would fit 6mm just to be sure. Dont have my regs book handy so cant work it out. Some people will export the earth from the house supply depending on the supply type, I did on mine, but the best way it to TT the external installation by fitting earth rods.
Run in the wire armour from the house supply to the workshop and connect the cpc, external part of the cable, only at the house end. This will protect the cable. Connect the phase conductors at the workshop isolator and insulate the end of the wire armour cpc from any connections. Fit earth rods at the workshop and use as your earth connection.
If you go down this route get an electrician to do this for you as this will have to be installed and tested correctly to ensure that it will work and possibly save your life.


Ian