a few things i also just thought of,
a 4x4 shed will have a outside frame diameter of 4'7"x4'7" if you use 2X4 construction, plan for that, unless you dont want a true 4x4 inside dimensioned shed.
your doors should be hinged at the edge of your framing for a shed this small... if you do the traditional doors (some wall framed in and then a smaller shed door) you will wind up kicking yourself when you need to move things to shimmy your mower out
the shed picture that was linked by woodshop flunky is a garden shed, not a yard shed, a garden shed does not store machinery, therefor a 3 foot door can cut it, i know i would make a huge bloody door...
learn to properly frame a door no matter what. alot of homemade sheds i have seen are improperly supported and after two years of weather exposure start to sag and become off square, making it a pain to close and lock. your door frame and the support form of the door itself should be a firm but not snug fit, if you make it too loose they will swing open when you dont want it to, too snug you will want to shoot the bugger who built it...