Cardiac action potentials non pacemaker

What marks the beginning of Phase 0 in the non-pacemaker cardiac action potential, and how does it influence the action potential?

1: K + ion outward current, slowing the upstroke
2: Ca 2+ ion inward current, stabilizing the membrane potential
3: Na + ion inward current, causing rapid depolarization
4: Cl - ion inward current, initiating initial repolarization