Answer:
Batman's equation of this scenario is not accurate.
Explanation:
The exponential function representing this situation should be 

where a is the initial amount of baddies living at Arkham asylum, b is the rate at which enemies increased, n is the number determing the period of enemies increasing and x is time in years.
In your case, 
a=4 (initially there were 4 baddies living at Arkham asylum), 
b=2 (the population doubled),
n=1/2 (the population doubled every 2 years).
Then

Note then when considering the function 
 the population dubled each year, not every 2 years.
 the population dubled each year, not every 2 years.