Three to four inches should be fine for Oklahoma. I wouldn't be a bad idea to check with someone at your local Home Depot or Lowes though just to be on the safe side. You need to decide before this project starts if you want the step to be part of the same pour though. If so, you'll have to prep your step frame so that you can lay it on top of your still wet pad.
The first step should be to dig out your pad roughly 6 to 8 inches deep and lay about 3 or 4 inches of construction sand as a base. Once you've leveled this out, wet it down with a fog stream from the hose for about 15 minutes to compact the sand (sand compacts naturally when wet).
You should now have about 3 or 4 inches below grade for your pad. Build your frame for the concrete and put down a vapor barrier (a heavy sheet of plastic from the hardware store) to prevent fluid migration. Additionally lay down some steel mesh to prevent cracking. You might want to check the frame with a level to make sure rain water will run away from the house. That will prevent for example an ice sheet during cold weather.
Now mix up your concrete and start pouring. You'll probably need about 6 to 8 bags to finish the job. Add just enough water to make the wet concrete easy to manage when finishing. It should be a really thick paste rather than a soupy mess. The less water you use during mixing, the stronger the finished concrete will be.
Use a board that will reach all the way across your frame to level out the pad and do your finishing work. There are a number of specially designed concrete tools. Probably a good finishing trowel and an edge finisher would be a good idea.
Wet it down once or twice a day until it cures (28 days) and you're done.
I'm a little under the weather here and not exactly thinking right so someone make sure I gave him all the steps. Hope your project turns out well.
E-95
Edited to add: Just remember the good news. If you screw up the first time, you can always spend hours with a sledge hammer and start all over again!