Perfect fourth interval

Perfect fifth in musical notationThe perfect fourth interval consists of two notes with five steps distance. For example, C to F note will result in this musical interval. The perfect fourth include one more semi-step compared to the major third (M3) and two semi-steps less compared to the perfect fifth (P5).

The reason for the name “perfect” goes back to the Medieval. The unison, fourth, fifth and octave were considered most consonant and therefore were given the name perfect. The perfect fourth interval is abbreviated P4 (an alternate spelling is augmented third).
Keyboard perfect fourth interval 
A perfect fourth with C to F on the keyboard.

Listen to perfect fourth interval (C-F):

This interval is rare in chords, but can be found in suspended 4th chord. For example, Csus4 is built by the notes C, F, G. If inversions are considered, they are much more common. For example, the second inversion of F major which is spelled C, F, A includes a perfect fourth.

The major third has on the other hand a key role in chords. It is the third that decide if a chord has a major or minor quality. Chords with a major third interval between the first and second notes are in general major chords. For example, C consisting of the notes C, E, G and Cmaj7 consisting of the notes C, E, G, B.

Ascending perfect fourth intervals

A list with ascending perfect fourth intervals:

Inversions of these intervals will result in perfect fifths intervals.

Descending perfect fourth intervals

A list with descending perfect fourth intervals:

Enharmonic notes such as Cb and Fb are leaved out.

Augmented fourth (tritone)

Augmented fourth fifth in musical notationA related interval is the augmented fourth, or tritone (abbreviated TT). It includes one semi-step more than the perfect fourth interval. For example, C to F#.

Listen to perfect fourth interval (C-F#):

Keyboard augmented fourth interval 
An augmented fourth C to F# on the keyboard.

Notice that the diminished fifth is identical with the diminshed fifth.