4 Ply Silk
One ply, two ply, three ply...Technically ply only becomes impractical when the yarns get two thick to weave. Ply has several meanings, depending on the context, i.e. plywood, or as a verb, to work diligently. Within the textile industry, a ply is "the number of single yarns twisted together to form a ply yarn or cord." (Fairchild, p. 466).
Now with silk, as the yarns are filaments, they don't technically need more than one ply before being woven. But, additional ply to the filament (meaning more than one spun together before weaving) adds considerable strength to the fabric. It's by adding ply that we get fabrics like crepe and taffeta and georgette. And with as drape-y and luxurious as silk tends to be, additional ply can make for some very slinky, luxe fabric.
Like four-ply silk. Four-ply silk is woven with four-ply yarns on both the warp and weft. This makes for a very heavy, smooth surfaced, gracefully draping, silk fabric. Because of the four-ply yarns, four-ply silk tends to be heavier than your average silks found in a shop, weighing in at 30MM to 40MM. The surface tends to be very smooth and thus this heavy fabric can be difficult to work with. Additionally, due to the additional ply, four-ply silk won't crease the way habotai or china silk will. Because this fabric is so heavy, it will also not gather as well, creating a great deal of bulk around gathering stitches. Julie Parker (p. 34) recommends garments that are shaped in the seams, versus gathers, pleats or darts.
Whatever you decide to make, it is silk, it will dye beautifully, and it will drape like a dream.