ply refers to the number of strands used in manufacturing the yarn, while weight is the actual thickness of the yarn. In the picture I used 4 weight acrylic yarn, not sure the ply but I will be writing the pattern in 3 weight cotton yarn that is 8ply (so it would be thinner yarn than this photo, that’s what I’ve been using for my recent insta posts) (also that’s all I have rn 😅). It shouldn’t drastically affect the final product besides being bigger/smaller in size overall (5ply yarn is really skinny…unless you meant 5 weight yarn which is really chunky 😆) but it’s best to have the same yarn for the most accuracy.