Mark 1/2" from the end of each 4x4.

Transfer the line to two of the other sides in a similar manner to step 1.

Set your circular saw blade to a 45 degree angle as seen in photo 2.

Bevel 3 of the sides by using the angled blade to cut along the line. The result will look like photo 3.

The fourth side does not get beveled because it lays flush against the wall.