If
\(\int x^3 e^x \, dx = e^x(px^3 + qx^2 + rx + s) + C\),
then find the value of \(p + q + r + s\).
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For integrals of the form \( \int x^n e^x dx \), use repeated differentiation of the polynomial (tabular or shortcut method) instead of full integration by parts each time.