The vertical lines are absolute value symbols. Basically, |x| measures how far x is from zero. You can also think of the absolute value signs as erasing the sign information. That is, |x| is always non-negative. The derivative of |x| exists everywhere except at x = 0. So the derivative of |x-4| exists everywhere except at x = 4. Since that's in your interval of interest, that'll be a critical point.
Can you finish from here?