This is the single most important geometric fact in P4 tuning. Memorize it and you can find any note on the entire fretboard.
The Rule
In fourths tuning, an octave is always 2 strings up and 2 frets higher. If a note is on string S at fret F, the same note one octave higher is on string S+2 at fret F+2. No exceptions. No special cases.
This works because each string is 5 semitones apart. Two strings = 10 semitones. Add 2 frets (2 semitones) and you get 12 semitones = one octave.
Why Standard Tuning Breaks This
In standard tuning (E-A-D-G-B-E), the interval between G and B is a major third (4 semitones) instead of a perfect fourth (5 semitones). This means the octave shape changes depending on whether you cross that boundary. Sometimes it is 2 strings up / 2 frets over, sometimes 2 strings up / 3 frets over. You have to remember which strings you are on. In P4, you never think about it.
Octave Map
Choose a note below to see every instance of it across the full fretboard. Notice the diagonal staircase - every pair of same-colored dots follows the same shape: 2 strings up, 2 frets right.
Pick any pink dot, go 2 strings toward the high F string and 2 frets toward the body - you land on the same note, one octave higher. This works everywhere on the neck, for every note.
Practice
Pick a note on the low E string. Find its octave on the D string (2 strings up, 2 frets right). Then find the next octave on the C string (2 more strings up, 2 more frets right). You have just chained three octaves diagonally across the neck. Click through each note above and trace the diagonal with your eyes until the pattern feels automatic.