'Capo 4' means to put a capo across the 4th frets. This merely shortens the string length and raises the tone of your guitar by two whole steps (4 half steps). You then play the same chords above the capo and they sound that much higher. Capos are typically used to alter the key of a song and make it easier to sing it in an more comfortable key.
I looked at the two version of 'Who Am I' here on chordie and did not see any indication that the guitar should be retuned to a non-standard tuning. Realize that songs out on the net have been tabbed or transcribed by people doing their best to figure out a song . . . sometimes there are all sorts of misunderstandings or mistakes on what you find.
Regarding the dark circles - if you mean the chord grids, then that merely indicates where each string should be fretted.
Hope this helps. James
"That darn Pythagorean Comma thing keeps messing me up!"
[url]http://en.wikipedia.org/wiki/Pythagorean_comma[/url]