We need to simplify the given expression step-by-step. First, convert mixed numbers into improper fractions:
\[
3 \frac{1}{4} = \frac{13}{4}, \quad 4 \frac{1}{3} = \frac{13}{3}.
\]
Now substitute these values back into the equation:
\[
\sqrt{\left( \frac{13}{4} \right)^4 - \left( \frac{13}{3} \right)^4} = \sqrt{\frac{28561}{256} - \frac{28561}{81}}.
\]
To solve this, first calculate the values of each fraction. Once simplified and after subtracting the two fractions, you will get:
\[
\sqrt{\frac{5}{12}} = \frac{5}{12}.
\]
Thus, the final answer is \( \frac{5}{12} \).