Walking Ballista has at least two +1/+1 counters on it, or your life total is at least 4.
Steps
Give Walking Ballista lifelink, if needed.
If Walking Ballista has less than two +1/+1 counters on it, activate Unspeakable Symbol by paying 3 life, putting a +1/+1 counter on Walking Ballista.
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 4 damage to an opponent or a permanent an opponent controls, and causing you to gain 4 life.