SUB. CHORDS CAN BE USED IN THE MAIN CHORDS OF YOUR SONG, THE TRICK IS WHICH CHORDS CAN YOU USE! DEPENDING ON THE SONG, rule of thumb, what you use for the 1, you can use for the 4.
lh / rh
CG / Em7 = 1
FC / Am7 = 4
YOU CAN ALSO USE THE 6TH AS A 1 CHORD IF YA DO IT RIGHT, HERE'S AN EXAMPLE IN F:
"NORMAL WAY"
LH / RH
GD / Bb
AE / C
BbF / Db
CG / Eb
FC / Am7
"THE 6TH SUB. ENDING"
GD / Bb
AE / C
BbF / Dm7
AE / GM7
DA / BDbEGbA
You hear the difference? That's a typical Kevin Bond move he does on the regular, instead of using the 1, he majored the 6th, you can sub the 1 with a minor 6, all he did was make it a M7th
hope this helped you out