Standard tuning needs five different fingering shapes to play the major scale across the neck (CAGED). Fourths tuning needs one.
Every string pair is separated by a perfect fourth. That uniformity means a shape that works on strings A-D works identically on D-G, G-C, and C-F. No special cases, no memorizing which string pair breaks the pattern.
The 3NPS Shape
Here is C major, one octave, starting from fret 3 on the A string. The shape uses three notes per string across three strings.
Look at the pattern on each string pair. The A and D strings both play frets 3-5-7: whole step, whole step. The G string shifts to frets 4-5-7: half step, whole step. That shift happens because of the half step between B and C (scale degrees 7 and 1). The major scale has two half steps - between degrees 3-4 and 7-1 - and where they fall determines the fret spacing on each string.
Why One Shape Covers Everything
In P4 tuning, moving up one string is the same as moving up 5 frets on the same string. The fret pattern on each string depends only on which three scale degrees land there. Since the interval between every string pair is identical, the geometry stays consistent across the entire neck. That is why one shape is enough to play the major scale in any position.
Practice
Start by playing the shape ascending (A to G string) and descending. Once it feels comfortable, shift the whole shape to a new root: fret 5 for D major, fret 7 for E major, fret 0 for A major. The fingering is identical every time. Set a metronome to 60 BPM with eighth notes and focus on keeping an even tone across string crossings.