This is a guide for cutting a 4x4 post with a hand saw (or power tool, I suppose). I used it to cut off the tops of my fence posts.
Basically you just slide it down the post to the desired level, clamp it, and then run the saw along it. If printed properly (see instructions) the top will be very flat, and you can place a level on it to get things lined up right.
The slots at the corners allow it to flex a little bit, so it can slide around any twists in the posts.
Another way to use this part would be as a guide to slide a circular saw along. That is actually what I designed it for. However in the end I found that each post only took a few minutes with the handsaw, and didn't require a second person to catch the falling post while I dealt with a power tool. And I only had 10 posts.
The saw will chew away at the plastic, so you might need to print more than one, if you are particular (my first print lasted 8 posts, before it cracked while I tried to hammer it down an extremely warped post)
Designed in Autodesk Fusion 360 (.f3d archive included)
I printed in PLA at low quality defaults in Makerware (.3mm layer height). This is a relatively large part, your build area needs to be at least 140mm/5.6 inches square).
Print in the orientation in the STL file, with the cutting surface pointed upwards. This will ensure that it is flat, and does not suffer from any warping due to the bed and/or curling.
There are mouse-ears at the corners of the base, they will slice off easily with an Olfa or Exacto.