Determine the height at which you want the tops of the posts to be cut off. Measure and mark that height at the two end posts.
Stretch your chalk snapline between the two end posts, so the string crosses each of the posts in between. Snap the line.
At the first post, use a tri-square and pencil to extend the horizontal chalk line from the one marked side of the 4-by-6 to all four sides.
Measure your circular saw, from the blade to the far side of the plate (the part that sits flat on whatever you're cutting). Then measure down from the chalk line on the post at that distance, and make a mark. Extend the mark around all four sides of the post, using your tri-square and pencil. You should end up with two parallel sets of horizontal lines going all around the post, at the same distance from each other as the width of your circular saw plate.
Set a piece of scrap 1-by-4 underneath the bottom line on one side of the post, so the edge of the board lines up with the line. Use your screwgun to sink two wood screws into the board to hold it in place. Repeat for each side of the post, so you have four scrap boards set up under the bottom lines.
Run your circular saw sideways along each side of the post, with the edge of the saw plate resting on the side of the scrap wood and the blade on the top line. The top piece of the post should fall off after the fourth side is cut.
Remove the scrap wood from the post, using your screwgun. Repeat the process for each post.