Este vorba de cvarta sau cuarta de sprijin armonic.
Referințe în engleză:
https://books.google.com/books?id=ed8hAwAAQBAJ&pg=PT31&lpg=P...
“sprijin armonic”
http://www.alandmoore.com/blog/writing-vocal-harmonies/
“I always figured 4th were safer since most chords have perfect fourths and fifths it was safer and supportive to sing the lower fifth.”
http://www.britannica.com/art/harmony-music
“The concept of harmony and harmonic relationships”
“These intervals, which occur in the music of nearly all cultures, either in melody or in harmony, are the octave, the fifth, and the fourth. (An octave, as from C to the C above it, encompasses eight white notes on a piano keyboard, or a comparable mixture of white and black notes. A fifth, as from C to G, encompasses five white notes; a fourth, as from C to F, four white notes.)”