how do the waves on an ECG relate to the different ion channels?

  1. this picture here makes it seem as though the P wave represents phase 4
    while the QRS complex represents the rapid depolarization due to the sodium channels (phase 0). Is this correct? if so why is phase 4 represented as a flat line?

    or is it that each wave on an ecg represents one full cycle (phase 4---phase 3)