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. Perfect fourth is “perfect” in the sense that is almost sounds as the unison interval. 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 the 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.

To being able to distinguish this interval by ear, a good idea is to think about familiar songs which first notes match. P4 examples are "House of the Rising Sun" (B-E) and "Moonlight Sonata" (G#-C#), in both cases ascending.

Ascending perfect fourth intervals

A list with ascending perfect fourth intervals:

Inversions of these intervals will result in perfect fifths intervals.

Below is an example with ascending fourth intervals in pairs:
First piano note example with fourths

Descending perfect fourth intervals

A list with descending perfect fourth intervals:

Enharmonic notes such as Cb and Fb are leaved out.

Below is an example with descending fourth intervals in pairs:
Second piano note example with fourths

Augmented fourth (tritone)

Augmented fourth fifth in musical notationA related interval is the augmented fourth, or tritone (abbreviated TT). It includes one additional semi-step compared to 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 augmented fourth is identical with the diminished fifth.