The actual kind of yarn is not mentioned. “4 ply” is deceptive, since that is not a universal standard. However, the requirement is for 64 oz of yarn, which is not very helpful, either, without knowing the kind of yarn required. The gauge might be more helpful. Worsted weight yarn has been called 4-ply in this group of magazines in the past.

Gauge: 9 dc = 4”, 4 rows = 3”. The entire afghan is worked with two strands of yarn held together. Use any yarn which gives you this gauge.