How to wire a sub panel from a 15 amp circuit existing run
You don't need a shutoff or subpanel for a single outdoor circuit. 

You should not put 4 GFCI receptacles outdoors if you can avoid it.   

The best way to handle the GFCI requirement is to find a place along the circuit still inside your dry house.  Put the GFCI there, whee itvwon't get destroyed by weather.   You know how to use the LOAD terminals on a GFCI to protect downline things, so do exactly that for your entire outdoor circuit.   

Now, because the wiring is also protected, you are allowed to use direct burial at 12" cover.   Though since you are using Rigid, you only need 6" cover.  You can use THWN individual wires in the conduit and do not need a ground since Rigid is the ground.