Converting Main Panel to Subpanel
Your plan sounds right. Separating the grounds.  And 4-wire from main to any subpanel.  Using aluminum wire for service and feeder is also wise. 

You actually need two ground rods at each building. Unless the one passes the magic test for 25 ohms. But you can't test it, and the cost of hiring it done is more than a second rod.  End of the day ground rods are for protecting you and yours, so they're cheap insurance.  

You still need the ground wire; that's for returning human-generated current, which is much lower voltage than lightning and ESD, but far too high current for dirt to carry.