It can be useful to know the prime factorization of the number:

Then, remember that the n-th root of a number is the same as that number to the 1/n-th power:
![\sqrt[4]{256} = 256^{(1)/(4)}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/2pu4bn92roeeemcwyfylyc1qvghzh42ti9.png)
Finally, you have to remember the rule for raising a power to another power, i.e. you multiply the exponents:

Put together everything we said so far to write
![\sqrt[4]{256} = 256^{(1)/(4)} = (2^8)^{(1)/(4)} = 2^{(8)/(4)} = 2^2 = 4](https://img.qammunity.org/2019/formulas/mathematics/middle-school/flwrp9085r1cjnq53u38mk83uvzrdxknet.png)