We can't answer the question without knowing what the initial temperature is. I can give you a formula to use, which will allow you to work out the answer.
Let T represent the temperature at any time, and T0 be the initial temperature (at 1pm), and let x be the number of hours past 1pm. Since the temperature drops 4 degrees every hour, we need to subtract 4 times the number of hours (ie, 4*x or 4x) from the initial temperature at 1pm, T0, thus:
T = T0 - 4x
Now to find x, the number of hours it'll take to reach -1 degrees, so when T = -1 degrees:
-1 = T0 - 4x
Now, rearrange to find x (steps: add 1 to both sides, then add 4x to both sides, then divide by 4, to leave us x):
-1 + 1 = T0 - 4x +1 =====> 0 = T0 - 4x +1
0 + 4x = T0 - 4x + 4x +1 =====> 4x = T0 +1
4x/4 = (T0 +1)/4 =====> x = (T0 +1)/4
Now, just put it whatever the initial temperature is at 1pm, and solve for x, the number of hours it'll take to reach -1 degrees.