Fourth
A fourth is an interval of four letter names, such as C–F. A perfect fourth is five half steps.
From C to F there are four letter names, both ends included: C, D, E, F. That is a fourth. On the staff one note sits on a line and the other in a space, with one line and one space skipped between them: no longer neighbours, but still close.
A fourth is not called major or minor: from C to F is five half steps, and that is a perfect fourth. D–G, E–A and G–C are the same size. The one exception among the white keys is F–B: six half steps, an augmented fourth, which sounds tense.
The fourth links the tonic and the subdominant: in C major, from C up to F is a fourth, which is why the fourth degree is called the subdominant. You can recognise the fourth from Wagner’s bridal chorus (“Here Comes the Bride”): its first leap is a perfect fourth up.
What it sounds like
A fourth sounds firm and open, a little stern – like a signal or a fanfare. It is close to the fifth, only narrower.
Where it is taught: the studio’s interactive textbook, chapter 15, “Intervals: unison to octave”.