make ground 4 gauge wire from several twisted 12 gauge solid copper wires
#4 has a area of 41,742 CM (Circular Mils).

#12 has a area of 6,529 CM.

As Dave Tweed says in the comments, you need 7 (6.39) connected in parallel.

Parallel conductors are permitted by electrical codes for phase, neutral or ground for #1/0 and larger.  Each conductor must be same length, material, insulation, area and termination.  The parallel conductor rule is set up for currents > 250A.

As Peter Bennett says, an electrical inspector will probably take exception to this because #4 is readily available.