I need to evaluate $\displaystyle \int sin x cos^3 x dx$

I tried 2 methods: u-substitution and integration using trigonometric identities but ended up with two different answers. Both workings look fine to me. Could anyone pls help out?

Method 1: U-substitution

$\displaystyle u = cos x$

$\displaystyle \frac{du}{dx} = - sin x$

$\displaystyle \int sin x cos^3 x dx$ = $\displaystyle \int sin x. u^3 \frac{du}{-sin x}$

$\displaystyle \int sin x cos^3 x dx$ = $\displaystyle \int - u^3 du$

$\displaystyle \int sin x cos^3 x dx$ = $\displaystyle \frac{-u^4}{4} + C$

$\displaystyle \int sin x cos^3 x dx$ = $\displaystyle \frac{-cos^4x}{4} + C$

Method 2 in new post.