The expression you provided is \( x - 4y^3 \). It looks like there might be a misunderstanding in looking for a simplified form, as this expression cannot be simplified further into a different form (like a fraction or a product) without additional context or operations.
However, if you're looking at it as a polynomial or an expression that could represent something, none of the responses you've provided directly match a simplified form of \( x - 4y^3 \).
If you wanted to simplify or manipulate further, please provide additional context or clarify what you're specifically looking for. If you're looking for a specific operation or transformation of the expression, let me know!