4. Among the following four figures, it is known that one is the graph of the equations $m x+n y^{2}=0$ and $m x^{2}+n y^{2}=1(m \neq 0$, $n \neq 0)$ in the same coordinate system. It should be
A
B
C
D
When $n>0, m>0$, the parabola opens to the left. In option $D$, $\frac{1}{m}>\frac{1}{n} \Rightarrow n>m>0$ $\Rightarrow \frac{m}{n} \in(0,1)$, at this time the parabola opens less widely, which is a contradiction. Therefore, the answer is $A$.

