We start with the expression \((4y^{2}+x)(y^{2}-3x)\). To expand this, we apply the distributive property, multiplying each term in the first binomial by each term in the second binomial.
Step 2: Combine Like Terms
After expanding, we combine like terms to simplify the expression. The result of the expansion is:
\[
-3x^{2} - 11xy^{2} + 4y^{4}
\]
Final Answer
The expanded form of the expression \((4y^{2}+x)(y^{2}-3x)\) is:
\[
\boxed{-3x^{2} - 11xy^{2} + 4y^{4}}
\]