6" won't hurt and as I stated earlier its what I have in my floor. Not really necessary unless you're going to be parking heavy duty trucks and equipment on it but Viper owners are all about extreme anyway

. Freeze/Thaw is a concern for concrete sidewalks and car port slabs but less of an issue for covered garages with foundation walls. I assume you are talking about a detached garage with foundation or stem walls that will be framed and covered? If you're worried about strength with a 4" then go with 4000 psi. The only cavaet I have with 4000psi concrete is that if someday you decided to expoxy your floor it will be a B**** to shot blast the floor for surface prep. Most contractors don't put rebar in a garage floor and go with wire mesh which I believe does nothing for a garage floor with load on it.
At least your contractor went with rebar. Did they specify how far the spacing will be 12" or 24"? 1/2" rebar? How about a vapor barrier? Did your contractor say he was going to lay expansion joints around the perimeter of the floor between the concrete slab and fondation walls? Is he going to score the concrete or install expansion joints in sections? It's OK to pour the whole floor then score it afterwards. So long as he does one or the other. If your garage has been filled in, what did he use for fill material?
You got good advice about the use of water (or limited use) and cold weather. Although its OK to pour concrete in cold weather, even freezing weather, there's a lot of prep and additional work involved to make this happen and there's a risk of the concrete becoming weak. The concrete temperature must be kept above 50 degrees for 5-7 days for a good cure. Contractors often use accelerating curing agents when pouring the concrete in the winter, then cover with blankets and/or use heaters. This all adds up to addtional cost and if the contractor is not a pro at this the concrete will be weak.
I forgot to add earlier that I mixed synthetic fiber in my concrete mix to eliminate shrinkage cracks but this was an experiment and not needed for my floor with all of the rebar. I'll go back to step 1...proper subgrade preparation. Make certain you've got good fill material and its been compacted. I replaced my neighbor's garage floor 2 years after his house was built. It began to sink and crack and became uneven. After we jack hammered the floor out we found no rebar, no mesh and the garage was filled with loamy material instead of sand and gravel. It wasn't compacted and compression tests on the concrete showed less than 3K psi which was probably caused from way too much water in the mix and it was poured in the winter and froze before it was fully cured. The floor was basically sitting on a sponge.