...level 4 is always in the key of C, and for this song, the key of C may either be hard to sing or the the most popular version of this song may be in a different key. In order to play in a key that people can sing along with or in the key where you can play along with the original, you're going to have to play level 4b. Here's what to watch for:
1 - Key Signature
At the very beginning of the song, you should see at least 1 sharp or flat symbol. If you don't already know the names of the lines and spaces, you'll need to look up the name of the line or space the sharp or flat is on, and then play that sharp or flat every time it appears in the song. For example, if at the very beginning of the song there is a sharp sign on the line reserved for F, then every single time the song says to play an F, you will actually play an F sharp. Every single time. Every octave, every hand, every clef, every measure - all F's are sharp.
2 - Accidentals
Throughout the song you may find other sharps, flats, or natural signs where they didn't show up in level 4. That's fine, just play each one as it comes up.