If the vertex colors are transparent black (r=0f, g=0f, b=0f, a=0f), then your image will not show. Setting the alpha component to zero would be the equivalent of rendering an image with 0% opacity (i.e. invisible). Setting it to 1.0 would be the equivalent of 100% opacity (i.e. fully visible).
Also, the red, green and blue components should be white (1.0f) unless you want to tint your image a different colour.
By this logic, if you wanted to render your image with 50% opacity:
glColor4f(1f, 1f, 1f, 0.5f);