for x > 4:
and its derivative is: 2x-1
and its derivative is: 2x+1
lim(2x-1) for x approaches to 4+ is 7;
lim(2x+1) for x approaches to 4- is 9;
so y's derivative is not continuous at x=4, thus y is not differentiable at x=4.
may this help you.