I mainly use a visual cue, and it's based off when the opponents body hits the ground.
You hit 2 almost exactly when the body hits the ground (first hit)
You hit 4 a split second (really a tiny fraction of a second) after the body hits the ground the second time.
You hit 3 a little bit longer after the body hits the ground, more like a moment after rather than a split second.
It's like each time the body hits the floor, you wait a few frames longer before hitting it again. Play around with that idea/timing and just practice it.