When I was doing our new shop, the concrete floor was one if not the most important single operation. If you want a floor that last and doesn't have 4 million cracks then it should be atop your list of most important things also. For a work shop 4" floors are just not thick enough, 6" should be the minimum and anywhere you know you will be doing heavy work, the floor should be thicker.
I took the time to lay everything out on our building, and I knew exactly where the lifts would be, welding table and area, storage racks for metal (which easily weight more then 15-20 tons!) , and made sure we reinforced these areas right. The 2-post lift area I made a 8 foot wide 12 foot long 16" trench to support the lift where it bolts to the floor. This gave the lift and me peace of mind and the confidence to work under my truck with no worries. Same goes for the area where I store steel, he have a back to back industrial style storage rack that is 12' high x 20' long x 6' wide. It stores mainly angle, round and square steel tubbing, and it adds up fast, right now we have about 15-20 tons on the rack. I have another rack with 10 tons of steel at another site. The main thing to take from this, is be prepared and do your home work, then there will be no surprises.
We had planed on a 6-7 " floor thru out the building, except in the before mentioned places. Due to a framing area, we ended up pouring the floors 4" thicker, which is fine and gave me more strength, but the 10-11" floors over all also cost another 4000-4500 more then the 6" floor.