What is the 4 or 4th Scale Degree in music?

The fourth scale degree is the fourth note above the root in a given diatonic scale. This scale degree has a "perfect" quality against the root. 

The opening notes of the Wagner's Bridal Chorus are root, 4, 4, 4...

When played as a harmonic interval. Perfect fourths can be found in the intro to Smoke on the Water by Deep Purple.