how do I find log base 256 = 4

OR maybe the asker may have meant

$$\\log_{256}x = 4\\\\
x=256^4$$

$${{\mathtt{256}}}^{{\mathtt{4}}} = {\mathtt{4\,294\,967\,296}}$$

We have log₂₅₆ 4

We can use the "change of base" rule to evaluate this

It says that  log_a b  =   logb / log a

So  log₂₅₆ 4    becomes   log 4 / log 256    .......and you can do this one on a calculator.....