How can I cut 4x4 posts that are already mounted?

I wouldn't use drawn lines to follow. Instead, I'd clamp a piece of scrap (?) wood to each of the sides, so it produced the correct angle - better still, use two longer pieces that straddle both posts.

Using an ordinary hand saw, cut from higher to lower, using the scrap as guides. A bubble will ensure both pieces are level with each other, when clamping.

The "idiot" who installed those posts did it using the normal way of installing posts. You always install them long then cut to length after they've been set. That way, you only have to worry about getting the post vertical during installation, and you don't have to worry about getting a pre-cut post to exactly the right height while trying to juggle 2 other dimensions.

Use a level to draw lines on 3 sides so that you can ensure your cut is level. You'll need this no matter what method you choose to cut the posts.

• A hand saw will work just fine to cut them and won't take long or cost much.
• You could purchase a circular saw and make 2 horizontal cuts, but if you're not used to using a circular saw, this is a good way to mess up your cuts and potentially remove bits of flesh from yourself.
  • I'd suggest not buying a "cheap" one. A reasonable quality tool doesn't cost that much more, will work better, and last much longer.
• You could use a jig-saw with a longer blade to make it through in a single pass, but jig-saws are known for not cutting the straightest lines, and since this is (presumably) right by your front door, I'd imagine you'd like a nice looking post top. You could hide the slightly ragged/not level cut under some sort of post cap.

It's common practice to set posts and trim them to height later. It's more difficult than you might think to set a post to a perfect height, and you don't always have the angles in advance..

Our strategy was to strike a line all the way around using a carpenter's square, then cut from all sides with a standard circular saw (four sides for level cuts, two for angles, or all four if you set the table angle). Be careful to hit the marks accurately. This leaves you with about a 3/4" square or strip in the center that you'd cut with a hand saw or reciprocating saw.

If you do this carefully there's not much left to clean up. If you miss a bit you'll have some sanding to do.

A hand saw may be a better option if you are cutting the posts at an angle. I would recommend a cross cut saw as a hack saw is usually for metal and may not cut completely through especially at an angle. A 12 point per inch 24-26” hand saw is what I use when I get in close quarters like the post next to the wall. Battery saws are great but not for every job.