Hey Joshua, I don't do concrete work any more, but I just had a slab poured for a patio at our business. It was 23x18. Total cost out the door was 2600. That included labor, material, haul off of material. He used a bobcat to regrade the back yard and remove material. He also used the bobcat to haul the cement from the alley to the patio. The cement was delivered and the driver sat there for about 1hr while he unloaded. You could also try texstar, they mix the cement on site and are sometimes cheaper. 4" is more than enough for a patio, anything more and your wasting money. Make sure that they also but atleast 3" of good base material and pack it. Also #10 or #8 WWF (welded wire mesh) should be fine. Make sure that they dowel into your foundation to prevent separation from your house. As for the slope, a slope of 1/4 inch per foot is what is recommended. On my 18' run, I did 1.5" and haven't had a problem with water. When the contractor sets the forms he takes this into account. Good luck, let me know if you need any more help.
Henry Moncada
"Courage is fear that has said its prayers"