I guess you could follow this procedure:

for x > 4:

and its derivative is: 2x-1

for x<4:

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.