Olav Torvund's Music Theory for Guitarists explains that "The perfect fourth is two and a half notes above the root,"
and provides audio examples of perfect fourths in the C major scale, which include C-F, D-G, E-A, G-C, A-D and B-E. The ratio of frequencies between the notes in a perfect fourth is approximately 4:3.
The inversion of a perfect fifth is a perfect fourth, and vice versa. For example, if the root of the chord C-G (a perfect fifth) is moved up an octave, it becomes G-C. As tdent notes, "There's also an old debate as to whether a note a 4th above the bass should be considered a dissonance (since it resolves to the 3rd above)."
The perfect intervals are a perfect fourth, a perfect fifth, a perfect octave, and a perfect unison, which is a chord using two of the same notes. Of these, the perfect fourth and fifth have a shared peculiarity: "If either of these are
(raised) by a semitone then they are termed 'augmented'. If they are (lowered) by a semitone they are called 'diminished'." This is according to a lesson at the On-Line Guitar Archive (OLGA), which also notes that the main purpose of learning to distinguish different intervals is to develop perfect pitch.
Lessons on guitar chords and music theory can be found at http://www.torvund.net/guitar/Theory/ 03-intervals-continued.asp (Olav Torvund); http://cnx.rice.edu/content/m10867/latest/ (The Connexions Project) offers simpler lessons with visual guides;
the use of different chord intervals in part-writing and melodic composition is explained at http://cctr.umkc.edu/user/bauera/Guide.html;
and relevant lessons from OLGA can be found at http://www.olga.net/dynamic/browse.php?local=resources/lessons/country/IntroToTheory.txt, which also features a large chord dictionary and information from Usenet guitar-related newsgroups.