Rewiring Outdoor Light to Switched Outlet (half hot, half not)
As you may have suspected, you would need four wires to have a half-switched and half-hot outlet.  Ground, Neutral, Hot and Switched Hot.  So, you could replace the light with an outlet, but it would have to be all switched or all hot (remove the light switch).

This day and age, you could use a regular outlet and then plug in an outdoor wifi switch to it and use that to switch something while the other receptacle always has power.