Please see my comment to the question.
Could you pay just a bit more attention to what Youtube itself tells you? Load the video on Youtube site using, for example, the URL you used in your
src
attribute. (No, this is not what you need to do.) I'm doing it with you. Now, it's loaded. Right-click your mouse on the video screen area. (I don't know your mouse settings, maybe for you this is left button click; in correct terms, use context menu click or keyboard key.) You will see the context menu, use the item "Get embed code".
The code I got is this:
<iframe width="854" height="510" src="https://www.youtube.com/embed/S4AQ1nBDBE4" frameborder="0" allowfullscreen></iframe>
Can you see the difference? Can you see that the URL is different?
Take care.
—SA