Answer:4
Step-by-step explanation:
When you see a number outside a root, it means how many times can a number be multiplied in that numbers times. E.g when you see something like this ³√64, it means how many times can a number be multiplied thrice to get 64 and that's equal to 4, (4*4*4).
Another example is ²√9 and that means how many times can a number be multiplied twice to get 9 which is 3 (3*3).
So back to your question ⁴√256 means how many times can a number be multiplied 4 times to give 256 and that's 4 because 4*4*4*4 is equal to 256.