...the left hand is moving all over the place and we've usually thrown in some extra notes in the right hand too! In case you're skipping straight to level 4 without doing 2 and 3 first, here's how to work out the notes on level 4:
1 - Lines and Spaces
The head (the round part) of the note can either be directly on a line or right between lines (on a "space"). Each line and space represents a specific note. You don't have a cheat sheet on level 4 because you really should be working on memorizing the letters of each line and space for the high notes in the right hand (treble clef) and the low notes in the left hand (bass clef).
2 - Rhythms
At level 4, you really should be putting each beat exactly in its place, but in general, the rules are pretty much like this: the more stuff on a note, the faster it goes. Just a hollow circle by itself gets held out for 4 beats. Add a stem and it's only worth 2. Fill it in - now it's only worth 1 beat. Throw a flag or a bar on that stem: half a beat! Each additional flag or bar cuts the value in half, meaning you play it twice as quickly. So hang on to those hollow notes for a long time and zip right through those filled-in notes with flags and bars. (To really get in-depth on the details on how to correctly read rhythms, you can join Klopol Piano Academy for free or just learn the basics by watching this video.)
3 - Intervals
If you're already past level 3, you already know this, but but even faster than memorizing the letter for each line or space is by playing "intervalically." First, find out your first note, then just ask, "Is the next note up, down, or does it stay the same?" Then just go in the same direction as the notes on the page.
If the note is going from a space to the next space up or the next space down, you're going to skip over a note (this distance is technically called a "third", but I just call it a "skip"). The same goes for moving from a line to a line: it's a skip away. Is the next note TWO lines or spaces away? ...that's a double skip (technically called a "fifth").
4 - Chords
If you are one of those people who really likes to experiment or make things up, or if there's just a part of the sheet music that doesn't sounds quite perfect to you and you would like help changing the notes in a couple places, you can use the chords as your guide. Each section of the song has a big letter assigned to it. That is the chord (the set of just a few notes) that all sound good during that part of the song. You can use that small set of notes anywhere on the piano and it will sound good...or at least not bad...you'll have to try some of them out to see which ones you like added in and which are best left out.
Probably the only two really helpful tips are: (1) usually you will want the highest note on the sheet music (the melody) to be the highest note you play - add in chord tones below the melody, not above; and (2) the farther down (left) you go on the piano, the more the notes need to be spread out. If you cram a bunch of notes together way down low, it will sound muddy and ugly even if the notes are technically part of the appropriate chord.