Answer:
 49 m/s
Step-by-step explanation:
We don't know what your model is, so we'll solve this based on the balance of forces. Air resistance exerts an upward force of ...
 (4 N/(m/s))v
Gravity exerts a downward force of ...
 (20 kg)(9.8 m/s²) = 196 N
These are balanced (no net acceleration) when ...
 (4 N/(m/s))v = 196 N
 v = (196 N)/(4 N/(m/s)) = 49 m/s
The terminal velocity is expected to be 49 m/s.