In all fourths tuning system, you can play C major, A minor, F major, D minor in open position and use all available strings.
C major / A minor
In open position, you can play from E2 up to A4. All the open strings are used in the C major scale. You can play the scale using 4 fingers as usual, i.e. index finger on the first fret, middle finger on the 2nd fret, ring finger on the 3rd fret and pinky on the 4th fret. An easier way to play is to shift your left hand 1 fret up for string 1->5, use index finger on the 2nd fret. This way, you’ll play the scale similar to 2 notes per string of the pentatonic scale.
F major / D minor
Similarly, you can play F major or D minor scale using all 4 fingers of the left hand starting from the first fret. Or you can shift your index finger up 1 fret for the F,C,G,D strings.
G major / E minor
Notice that the open F string is not used.
