No Screen Name wrote:
Is it better to put #4 bar 18" on center or 6 gauge wire mesh in 8" thick slab that will see heavy industrial traffic?

Answer:
The answer is NEITHER. You either want to use #4 at 12" on-center or #5 at 18" on-center for an industrial traffic slab. 6 gauge wire is only 0.162 inches in diameter which, when welded at 6" on-center, gives you 0.0412 square inches per foot of slab. You need 5 times that reinforcing! #5's at 18" gives you 0.207 square inches and #4's at 12" gives you 0.200 square inches. Skimping on the reinforcing now will be very costly later when you have to rip out the cracked, shifted slab!