I'm not getting Wilson's theorem to work? Wilson's theorem states that:

p is a prime if and only if (p - 1)! $\cong$ -1 (mod p)

Obviously 5 is a prime so this should be true:
(5 - 1)! $\cong$ -1 (mod 5)
But when I tried to test it it doesn't work:
$(5 - 1)! = 4*3*2*1 = 24$
I get the results:
24 % 5 = 4
$24 \cong 4$(mod 5)
Edit: I'm so stupid

Do not worry, you are correct.
Note that  $$24 \cong 4(mod5) \cong -1(mod5)$$
Remember, $$24-(-1)=24+1 =25=5k$$
