SO4 has a charge of -2, so the oxidation number is also -2. Since the overall oxidation number is 0. Then let Cr =x.
x + (-2)2 = 0
x -4 = 0
x = 4.
I took the overall charge of SO4, am I supposed to take the charge of each element individually, or was my right?
You were correct.
Take whatever charge you can be most sure of. SO4 is always 2-. (On the other hand, S can have any number of different oxidation states, so you don’t want to start there.)