Lets look at the notes again.
In 1st position the C Major chord is the tonic or root or I chord of the C scale. In 2nd position the C Major chord is the sub-dominant or IV chord of the G scale, which is part of the I, IV, V blues progression. Notice that the 4 draw is the same as the 1 draw, a D note. Doubling a note does not change the chord, so drawing holes 1-4 also produces a G Major chord. For 1st position the G Major chord is the dominant or V chord of the C scale.