This is restorative work, using as much original material as possible.
Lower part of post is rot, get another post (4x4) (6ft). Cut out the lower 6 inches and cut a 6 in piece from the new post.
using 1 inch wooden drill bit to drill into the new section for a depth of about 4 inches (takes time ).
using a bit extension to drill a 4 inch screw into the hole and connect the new piece to the old piece.
caulk and paint.