I'm gonna be the engineering ass here.

Its concrete, not cement. Cement is the grey powder used to make concrete.

Concrete is about $150 per cubic yard. Going from 4 inches to 6 inches shouldn't cost you more than 1 cubic yard for most conventional garages. This price is for high quality concrete, ask for CDOT Class D concrete.

4,500 psi with a maximum water to cement ratio of 0.45 and a 5 to 8% air content is code for concrete in Colorado to resist freeze thaw damage. Most residential concrete contractors cheap out.

If exposed to salts, solvents or other corrosive materials you need a concrete with fly ash and low water to cement ratio to reduce permeability. 4500 psi concrete with these adds no cost with these. Fly ash also eliminates efflorescence and a chemical reaction with salt. Outside denver metro. Aggregates react with cement, class f fly af is needed to mitigate this reaction.

Joining should be a maximum of 2ft per inch thickness to reduce cracking potential. Joints should be at least 1/3 the depth of the slab. To really minimize cracking and hold any cracks right, add MACRO fiber reinforcement. 5 to 7 lbs of fiber per cubic yard. Adds about $10 per yard. Could also eliminate jointing and rebar.

Most important, the finishers should NEVER add water to the surface during finishing. Water increases the water to cement ratio at the surface which leaded to scaling/flaking of the surface when exposed to freezing or salts. Finishing should be minimal, over working concrete may make it look nice, but also cases the same thing for concrete exposed to salt or freezing.
Curing, should always occur. Cover with a wax based curing compound or wet burlap for 5 days. The burlap must be kept wet. If curing compound is used, it must be applied so that you see no grey.

Sorry if this insults any contractors here, but not following these or cheating out on materials causes the most distress to concrete in Colorado. You see it every day.