Well, that's exactly what it should match. Your input matches your requirements. It has a block of text consisting of between 1 and 4 (1 in this case) lines of not more than 40 characters and an additional newline.

Your requirements are poorly specified.

Maybe, rather than see if your input matches such a block, you want to see if your input is such a block. If this is the case, your example input wouldn't work even if the second line had less than forty characters because your third line doesn't have a trailing newline.

If that is what you are trying to do, try this:

/
^               # Anchor to beginning of string.
  (?:           # Group each line.
     .{0,40}\n  # 0 to 40 characters followed by a newline. 
  ){0,4}        # 0 to 4 lines.
$               # Anchor to end of string.
/x;             # /x for comments.
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-sauoq
"My two cents aren't worth a dime.";