Continuing with the music theory thread...

Here's how I think and teach the fourths, as long as you know your open strings you're good to go (fifths are the same thing in reverse):

If you think of a seven string guitar (standard + low b) you could do this: C, F then from B to G down a half step (Bb, Eb, Ab, Db, Gb), then from B to G again (B, E, A, D, G all naturals), repeat ad infinitum.

It probably sounds more complicated than it is, cause it sure works for me and my students so give it a shot