Gravitational acceleration assumed at 9.8 (m/s)/s
Mass (m) = 30 kg
If its still accelerating at 4 (m/s)/s at 70 m/s, then the net force on it = 30 * 4 = 120 Newtons
Since the constant force due to gravity = 30 * 9.8 = 294 N , then the remaining force must represent the air drag at 70 m/s = 294 - 120 = 174 N
The drag coefficient (Cd) can be derived from :
Cd = 174 / velocity²
Cd = 0.03551
At terminal velocity, m * g = v² * Cd
So, v = square root ( ( m * g ) / Cd )
v = 90.991 m/s (terminal velocity)