As we learned in the previous lesson, counting up four degrees from almost any note in the scale yields a perfect 4th.
This is true in any key, not just the key of C.
This yields a so called Augmented 4th because it is 6 half steps instead of 5.
For example, if we wanted to play a Perfect Fourth starting on Eb we would go Whole, Whole, Half and arrive at Bb.
That is, if one note is a sharp, the other note will almost always be a sharp as well.
The only exception is starting on F or F#, for the reasons mentioned before.