DIY lamp post using 4x4 pressure treated wood?
It may be easier to make a post out of two 2x4's where you can use a router to cut in a V groove down the center of the face of each piece. The two V grooves facing each other would make a channel down the middle of the post. 

You may want to consider the use of a PVC conduit in the channel to eliminate the possibility of rusting.

An easier procedure than cutting a the V groove would be to build your lamp posts out of four 2x4s like shown below (end view). This would result in a nice sturdy post that was 5" square with a 2" square hole down the center for the conduit. And if this was constructed from pressure treated lumber before it fully dried it will likely hold its shape as it dried out.