Measure 4 is where the Adagio movement of the Moonlight Sonata by Beethoven starts getting interesting.
It is also where the value of these videos starts to make sense.
There are 2 really common mistakes that people make when playing it. First, when there is a sharp in front of the "B" it is really confusing. Up until this point, for many students, all sharps are black keys. The idea of a sharp being a white key confuses students, even if they have been playing the piano for a while.
The second mistake, and it happens to experienced players as well, is forgetting that the sharps last until the end of the bar line, so both the B and the D remain B sharp and D sharp for beat 4.
As the harmony is less straightforward, it isn't as easy to catch a mistake in beat 4 of measure 4 as it would be in other places in this piece.