Well that is definately not what I am seeing, so I did a fresh Joomla 3.4.3 install and then the Metropolis 1.8 bundle - same error ( lack of 404 error page).
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Then I did a fresh install from the Rocketlauncher as I noticed that you were most probably using the demo template - same error.
Of course this is not just Metropolis - most if not all templates running Gantry 4 suffer the same error.
Again, search Google for "public_html/libraries/joomla/document/document.php on line 1018" and you will see how widespread the issue is
It would be interesting to fathom out how you got Metropolis to error correctly.