In Exercise 4, we'll be playing a neat little trick using open strings and legato.
The whole pattern of this riff will be a grouping of 3 notes. First you'll play the open B string, then you'll hammer-on a B string note, then you'll hammer-on "from nowhere" on a G string note.
But before you play anything, let's examine the rhythmic note values in bar 23. For the first 3 beats, you'll see that we'll be playing straight 16th notes. But then, what will we be playing on the 4th beat? As you can see in the tab, there are 6 notes in the 4th beat, so that means we'll be playing 16th note triplets (6 notes per beat).
Once you have the feel for the rhythm down, this riff will be a lot easier. Lastly, make sure you also watch out for unwanted string noise.