Answer :

[tex]f(x)=|x-4|\\ f'(x)=\text{sgn} (x-4)\\ \text{sgn} (x-4)=0\\ x-4=0\\ x=4\\\\ [/tex]

For [tex]x<4[/tex] [tex]f'(x)<0[/tex].
For [tex]x>4[/tex] [tex]f'(x)>0[/tex].
[tex]\Downarrow\\ [/tex]
At [tex]x=4[/tex] there's turning point, which is equal [tex]|4-4|=|0|=0[/tex].