RVTRKN, if I was pouring that slab I would go 6" in depth. At 6" you will need 22.5 yards and the additional expense over the 4" slab which will require 18 yards will be worth it. And, I say 22.5 verses 18 yards, if you are dead-on grade. Leave out the fiber; ready-mix companies are only selling 'the sizzle with the steak' because most only put a very minimal amount and you need 3 pounds to the yard to really establish any kind of toughness index. You will also need to either have the slab cut or grooved a minimum of one and a-half inches in depth to prevent random stress related cracking because concrete will shrink and then crack, so it's important to anticipate where it will occur and control it. A 6" slab, if the finisher doesn't pour the concrete too wet, will crack every 15-20' (concrete can shrink up to 1/8" every 20 feet). This can be somewhat circumvented by pouring low slump concrete (stiff and harder to work) but using a jobsite added super-plasticizer that would cost several dollars more. I would feel comfortable using 3000-psi concrete but if you anticipate your finisher fussing over having to place 4-5" slump concrete, use the 4000-psi material. When adding water to concrete you can figure that for every gallon per yard that is added above the mix design, the slump will increase about 1" and the strength will go down about 200 psi, the equivalent of 1/4 bag of cement.
Where would I cut or groove? Right down the middle and then make three cuts the other way.
Use some concrete brick which you should be able to get from your ready-mix supplier as chairs to keep the finishers from displacing the rebar when they are screeding and bull-floating the concrete. Reinforcement is worthless if stomped into the sub-base.
Whether you pour the slab on the ground or dig it in (I'm not familiar with your soil) insure that the sub-base is uniformally compacted. You may have to use some gravel.
And finally, use a curing compound. Concrete requires the presence of moisture to properly hydrate; it is a chemical reaction not just 'drying out' that gives it strength.
Good Luck! Ed