In some parts of the world, ply is used to denote thickness. See http://yarnforward.com/tension.html for the gauges. In the US it would be equivalent to fingering weight, 4 ply would be about sport weight. Baby yarn is usually fingering or sport weight. So try to find a yarn that’s got about the same gauge on the label and then knit it up in the pattern. Hobby Lobby’s Baby Delight or I Love This Yarn sport should work very well, and there’s some RH, Bernat and maybe Lion baby yarns. Depends on what sort of colors you want.