Answer:
The compression is 

Step-by-step explanation:
From the question we are told that 
 The first velocity is v 
 The compression of the spring is 

 The second velocity is 4v
 Generally according to the law of energy conservation 
 The kinetic energy of the block is equal to the energy stored in the spring that is 
 

For first speed 
 

=> 

=> 

For second speed 
 

=> 

substituting for k 
=> 

=> 

=> 
