OK, here's a thorough explanation:
Gauge refers to the thickness of the thread. #-ply to the number of threads that are used, as in the number of threads used in every single stitch: 4-ply means it's knitted with quadruple threads instead of a single thread.
Neither thickness nor number of threads is a measure of quality in itself, but it's a good guide. It would be great if you had both gauge/threadcount.
It's like this with cashmere and other wools: the fibers in lower quality wool are shorter, thicker, coarser and less flexible than in good quality wool. High quality wool fibers are therefore longer, thinner, softer and more flexible (elasic, if you will). This means you can produce thinner yarns from good quality raw material than you can from lesser qualities. And it's better to use several thinner threads compared to a thick one because you get a softer, more flexible knit.
4-ply cashmere made of 4 thinner, higher quality threads will be softer, more flexible and finer* than cashmere made from a thicker single ply thread. Even if they are made from the same raw material; we can compare it to paper. Say you had two stacks of 10 thin sheets of paper, one with loose papers (10-ply) and one where the papers are glued to each other (single thread). Which stack is easier to fold and manipulate?
So anyhow, what kind of threadcount and gauge you use is - of course - also a matter of design: you really can't make a thicker, bulkier sweater using multiple ply, superthin yarn. It would take an awful lot of yarn to make it thick which would make it very heavy, and it would be too soft to get bulky.
*Also more expensive because you get a tighter knit which takes more material (and time) to produce.