Cotona No.4 and Decora No.6 are threads for decorative seaming. Therefore, it is recommended to use those threads only with overlock machines and threw the gripper/hook, shown in the picture. Those threads can only be stitched out of the bobbins, the technique is called bobbinwork. In this technique you stitch from the back side of the fabric, as the threads is in the bobbin. In the top needle there is normal thread, or bobbin thread. The pictures show embroidery done using this technique.