Building a thick concrete pad
I think the better approach is to build a thicker gravel base, and use less concrete for the pad. Backfill with gravel, in layers, compacting between each 4"-6" layer. 

I'm not sure you're aware of how much concrete you'll need for your proposed 12" thick pad. See this: http://www.calculator.net/concrete-calculator.html. Your 4'x4'x12" pad will use 27 bags of concrete, nearly a ton, if buying pre-mixed. I think building a thicker base, and then pouring a 4" or 6" concrete pad, with some rebar mesh, is a better/easier approach.

Your plan is solid, but I'd add several things to is:


Definitely use a rebar mesh.   
Put a 6mil vapor barrier into the gravel, to prevent it sucking all the moisture out of your concrete pad. 
Add a release agent to your form, it will be easier to remove it.
Make sure your pad is not connected to the house foundation, especially if the soil freezes in the winter in your area.