Are you given that " a " represents a non-negative number?
If the instructions say that, then your answer is correct.

However, if the instructions do not say that, then it is 
possible that " a " can represent a negative number,

By agreement a radical representing an even root must
always represent a non-negative number.  Therefore
your answer should be 

             4|a|   

with absolute values around the " a ", so that the
even root radical  will only 
represent a non-negative number.

Edwin