Fourths can be perfect, augmented or diminished. Below you can see that the number of steps determines the quality of the interval:
Perfect fourth, 2 1/2 whole steps or 5 half steps:
Augmented fourth, 3 whole steps or 6 half steps:
Diminished fourth, 2 whole steps or 4 half steps:
When analyzing the quality of a fourth we should know that:
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 I > Intervals for related entries. To learn about go to Tutorials > Intervals.