Either way will work. The miter saw will probably give a cleaner, more square cut. In either case, unless you have a honkin' big saw, you will have to rotate the 4x4 to complete the cut. I have cut 4x4s by cutting across to the maximum depth with a circular saw, then turning the 4x4 90 degrees. Place circular saw so that blade enters the cut that was made, and make the next cut. Turn another 90 degrees, do the same. The cut isn't perfectly clean but it's pretty good; clean it up with a rasp if you need an absolutely flat cut.