Well let’s say we have $0 and this is purchased for the sake of this answer.
We would have $-4 and one slice would give us a profit of $2. This is rinse and repeat and then you would have to sell 600 pizzas.
Because then:
$6 X 600 = $3600
$4 X 600 = $2400
$3600 - $2400 = $1200
So you would have to sell 600 pizzas to raise enough money