SINGER 14T967DC-WORKBOOK QUANTUMLOCK - Manual (Page 127)

Four-Thread Safety Stitch
This stitch is an alternative to the Five-Thread Safety Stitch. It offers the same type of
Two-Thread Chainstitch. However the Three-Thread Overlock is now replaced by a
Two-Thread Overedge finish. The Four-Thread Safety Stitch is appropriately used
when sewing a medium weight woven fabric. Because there is one less thread being
used, the stitch is less bulky. However, it is still strong enough not to require straight
stitch reinforcement from a conventional sewing machine. This stitch requires two
needle threads and two looper threads.
Four-Thread Safety Stitch page 1