Consider the following premises. For all x_4, x_4 is not a janitor. For all x_4, x_4 is a mechanic or x_4 will work. For all x_4, x_4 is a mechanic if and only if x_4 is a janitor. Can we infer the following from them? There is at least one x_4 for which x_4 will not work.
 From the fact that for all x_4, x_4 is a mechanic if and only if x_4 is a janitor, and that for all x_4, x_4 is not a janitor we can infer that for all x_4, x_4 is not a mechanic. Finally, from the fact that for all x_4, x_4 is a mechanic or x_4 will work, and that for all x_4, x_4 is not a mechanic we can infer that for all x_4, x_4 will work, which contradicts that There is at least one x_4 for which x_4 will not work. Therefore, the answer is no.