Someone

            for ( int n = 1; n < 4; n++ ) {
                file = "640_" + n + "_";
                for ( int i = 0; i < 4; i++ ) {
                    if ( i == 3 )
                        file += "d";
                    else if ( i == 2 )
                        file += "c";
                    else if ( i == 1 )
                        file += "b";
                    else if ( i == 0 )
                        file += "a";
                    listBox1.Items.Add( file + "_loading" );
                    file = "640_" + n + "_";
                }
            }