\( \sqrt[6]{x} = x^{1/6} \).

   \( P = x^4 \cdot x^{1/6} \).

   \( x^4 \cdot x^{1/6} = x^{4 + 1/6} \).

   \( 4 = \frac{24}{6} \),  

   vậy \( 4 + \frac{1}{6} = \frac{24}{6} + \frac{1}{6} = \frac{25}{6} \).  

   Do đó, \( P = x^{25/6} \).