Just learning about wood and enjoying it quite a bit so far.
I think one rule of thumb that I have learned is that
laminating boards to get larger dimensions is ok.

I'm building a pretty nice mailbox and need a 4x4 cedar
post for the above ground part of the post. I'm having
trouble finding the post in my area so I'm just going to
glue (titebond 3 waterproof) four 1 inch cedar
boards together to get my post.

Two questions:

Is it a general rule of thumb that gluing up thinner dimensions
of wood is not a problem? When would it be a problem?

The mailbox plans call for a pressure treated post in the ground
and a cedar post above ground. All surfaces will be painted. I
assume that cedar is used because it will have a nicer finish than
pressure treated pine even when painted.
Does that sound like the reason cedar is used?

Thanks

Ken