Completely factor 

            

That's the difference of two perfect squares.  
The fourth power is "the square of the square". 
So write the expression this way
 
            

Then that factors as

           

But that's not finished yet, because what's in the first 
parentheses will factor some more.  It is the difference
of two perfect squares too.

      

That's the final answer!

Warning: Do not try to factor further!

The DIFFERENCE of two squares  factors but
the SUM of two squares, such as  does not 
factor.

Edwin