Try this option:
1) if V(0;0) and x= -4, then common view of the required equiation is:
(y-k)²=4p(x-h), where focus is in (h-p;k), the vertex is in (h;k), the directrix is x=h+p, p<0 and y=k is simmetry axis;
2) if the V(0;0), then h=k=0 and the required equiation is:
y²=4px;
3) if the directrix equation is x=h+p, where h=0, then p= -4 (according to the condition the directrix equation is x= -4), then the required equation is:
y²= -16x
answer: y²= -16x