\(\frac{x}{a}=\frac{y}{b}=\frac{z}{c}=t\Rightarrow\hept{\begin{cases}x=at\\y=bt\\z=ct\end{cases}}\).
\(4=\left(a+b+c\right)^2=a^2+b^2+c^2+2\left(ab+bc+ca\right)=4+2\left(ab+bc+ca\right)\)
\(\Rightarrow ab+bc+ca=0\)
\(P=xy+yz+zx=abt^2+bct^2+cat^2=t^2\left(ab+bc+ca\right)=0\)