4. The function $f(x)=x^{2}+x+a(a>0)$, if $f(m)<0$, then the value of $f(m+1)$ is ( ).
A. Positive
B. Negative
C. Zero
D. The sign depends on $a$
4. A. Axis of symmetry $x=-\frac{1}{2}, f(0)=a>0$, then the two roots $x_{1} 、 x_{2}<0, x_{1}+x_{2}=-1$.