You then need 5^x = 625.
So x = log(625) / log(5) ; got an "ln" key on your calculator?
If 4 is the given, then x^4 = 625.
So x = 625^(1/4).
Tattoo this on your wrist (under your watch!):
If a^b = c, then a = c^(1/b)
If a^b = c, then b = log(c) / log(a)