One popular method is to use 1×6 inch lumber ripped to fit around the post, with a spacer/nailer 4×4 inserted as an extension to nail the face boards into. Another easy way is to add metal fence post extender fasteners to both posts on the opposite sides – front and back or add a wood face board on the front and back of the existing post. You can also use wood connectors and purchase two sections of 1×4 inch lumber to extend the post half the length.