I agree with most here, I would go with 5" and rebar.  Fiberglass used to be cheap, we added it just to be on the safe side.  Don't know how much it costs now.

That said, I knew of many hog feeding floors with open sided pens back in the day before everything was confined that was poured with less than an inch of concrete.  The idea was that it would be short lived, and hogs and a skid steer didn't stress much.  I was shocked when I heard it, but realized we usually found other uses for all the concrete we ever poured, so we wanted to do the max possible strength.