keithk said:4" think concrete isn't thick enough for #4 rebar, you should have at least 2" clear on all faces of the concrete, 5" thick min concrete, 6" is MUCH better. Rebar needs to be at least 3" from the end of the concrete.
Bingo. I was about to say all these rebar recommendations were wrong, but you beat me to it. I'm an old concrete guy (thank goodness I don't do it any more!). We built our driveways with rebar in the approach and the remainder was wire. I hope you don't have anyone on your deck heavier than a car. Concrete mixed right, poured before it's hot (hot as in curing, not temperature), and installed correctly will be perfectly fine at 4" with wire for a deck.
As Keith said, too much rebar and too little concrete and you end up with a mess.