The next code block is similar. The only difference is that the primitive drawn is a line loop and so only four vertices need to be specified - the last one is automatically connected to the first.
glColor4fv( green );
glBegin( GL_LINE_LOOP );
glVertex2f( -0.5, -0.5 );
glVertex2f( 0.5, -0.5 );
glVertex2f( 0.5, 0.5 );
glVertex2f( -0.5, 0.5 );
glEnd();