4. Let $f(x,y)=x^2+4xy+y^2$. We call the region $\{(x,y)\mid x^2+y^2<1\}$ open disk $D_1$, $\{(x,y)\mid x^2+y^2\le 1\}$ the closed disk $D_2$, and $\{(x,y)\mid x^2+y^2=1\}$ the circle $C$.

i) $(10\%)$ On the open disk $D_1$ find, if there is any, those points on which $f(x,y)$ is a relative minimum, relative maximum or a saddle point.

ii) $(10\%)$ On the circle $C$ compute the absolute maximum value and absolute minimum value of $f(x,y)$.

iii) $(10\%)$ On the closed disk $D_2$ give your reasons to find the absolute maximum value and absolute minimum value of $f(x,y)$.