„This machine was one of the most complicated of the Cornely
embroidery machines but it produced a great variety of work.
It had two needles and two underthreads which made two parallel rows of chain stitches.
It was called a four-thread embroidery machine because a four-thread flat embroidery effect was obtained by means of a thread of silk, cotton, etc., which was wound round the two parallel lines of chain stitching and a cord.
Very often the cord was dispensed with but nevertheless this class of work was still known as four-thread work.
The machine could be operated with either one or two needles. When used with two needles the distance between the needles could be varied to produce four-thread work of different widths.
This was either used as decoration or as edging seams on curtains, ladies‘ dresses and petticoats. Two needles were also used to make a double picot stitch.
When regulated for embroidery with one needle this machine produced all the different kinds of work done on the A, B, k and L machines, that is: chain stitch, moss stitch, flat or raised braiding, two-and three-thread cording, gimp work and picot stitch.
This machine could even be fitted with needles to produce three parallel rows of chain stitches simultaneously.
The four thread work was particularly used as an edging stitch and, besides
the variation in width produced by the distance between the needles, could have a variation in bulk according to the thickness of the thread or cord which passed down the central tube.“
(Quote from Machine Embroidery a complete guide by Christine Risley)