If the amplitude of a wave increases by a factor of 4, how is the intensity changed?

The intensity increases 16 times.

The relationship between intensity and amplitude is given below:

   I = constant 

Since, A becomes 4 times, A' = 4A
Thus the intensity becomes, I' = constant × (4A)² 
                                              
                                         ⇒ I' = 16A²

Therefore, the intensity of the wave is increased by 16 times

It indicates no change in temperature and the process of matter changing its state