4=0

I present you my attempt to demonstrate that $4=0$ :

1) Beginning with the well-known identity $\cos ^{ 2 }{ x } =1-\sin ^{ 2 }{ x }$

2) $\cos { x } ={ (1-\sin ^{ 2 }{ x } ) }^{ 1/2 }$

3) $1+\cos { x } =1+{ (1-\sin ^{ 2 }{ x } ) }^{ 1/2 }$

4) ${ (1+\cos { x } ) }^{ 2 }={ (1+{ (1-\sin ^{ 2 }{ x } ) }^{ 1/2 }) }^{ 2 }$

5) Evaluate $x=\pi$ : ${ (1+\cos { \pi } ) }^{ 2 }={ (1+{ (1-\sin ^{ 2 }{ \pi } ) }^{ 1/2 }) }^{ 2 }$

6) ${ (1-1) }^{ 2 }={ (1+{ (1-0) }^{ 1/2 }) }^{ 2 }$

7) $0=4$

In which step is the first error committed?