Just like triads (tri- three) that are made of thirds, you can construct chords out of the perfect fourth interval.
Simply, and open fourth structure is a chord built from two P4 intervals on top of each other, spanning a m7 interval. A closed fourth structure is one that spans an interval of a P4 with a third below the melody note (either m3 or M3 depending on the diatonic scale)
However, in the case that the interval of the diatonic closed fourth chord is a tritone, the best way to reharmonize it is to create an interval of a third between the bottom note instead of the top:
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