[The Newell]

As long as you have good preparation before you pour, and use the fiber reinforcement, you should be ok with the 4" slab even though I would recommend a 6" if it were me. The problem with concrete cracking is when it is on poorly prepared soil and/or not reinforced properly. Concrete generally cracks when there is movement beneath it, or it's uneven somewhere.

But if it really concerns you, pour an extra foot near the rear where the bulk of the weight will be.

I'm not an expert on concrete, but I did stay at a Holiday Inn Express last night