Diminished fourth, 2 whole steps or 4 half steps
Perfect fourth, 2 1/2 whole steps or 5 half steps
Augmented fourth, 3 whole steps or 6 half steps
When analyzing the quality of a fourth we should know that:
- the interval is a perfect fourth if all the notes are natural, with the exception of the fourth F-B, which is an augmented fourth.
If there are accidentals you should identify the interval without accidentals and then analyze the effect of the accidentals.
Another example: C#-F#:
Identifying fourths by counting whole and half steps is slow and confusing. See Simplifying accidentals.