gradient

I'm drawing a rectangle like this:

    glBegin(GL_QUADS);
      glColor4f(color1.r / 255.0, color1.g / 255.0, color1.b / 255.0, color1.a / 255.0);
      glVertex3d(x2, h - y1, 1);
      glColor4f(color2.r / 255.0, color2.g / 255.0, color2.b / 255.0, color2.a / 255.0);
      glVertex3d(x1, h - y1, 1);
      glColor4f(color3.r / 255.0, color3.g / 255.0, color3.b / 255.0, color3.a / 255.0);
      glVertex3d(x1, h - y2, 1);
      glColor4f(color4.r / 255.0, color4.g / 255.0, color4.b / 255.0, color4.a / 255.0);
      glVertex3d(x2, h - y2, 1);
    glEnd();

I expected every corner of the rectangle would have another color (see, there's color1, color2, color3 and color4), but it doesn't work: instead, the whole rectangle has color4. What is it supposed to do? In the nehe tutorials, it works for the triangle with blue, red and green corners... Before drawing the rectangle, I already do the following OpenGL commands (It's a 2D engine btw):

    glShadeModel(GL_FLAT);
    glCullFace(GL_BACK);
    glFrontFace(GL_CCW);
    glClearColor(0, 0, 0, 0);
    glViewport(0, 0, screenWidth, screenHeight);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, screenWidth, 0, screenHeight, -1024, 1024);   

    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_BLEND);
    glDisable(GL_TEXTURE_2D);