Sorry for your problems and I wish that I did not have to Post-Critique someone else’s work that I looked at, so please let me know if my opinion comes off as biased and out of character from previous critiques.
The 4-ply is Extremely more resilient. That additional ply counts as an actual additional layer of adhesive and another veneer of sheathing applied to it.
The structural and dimensional stability of the 4-ply is akin to comparing the 3-ply to 3/8" plywood.
For the reasons you stated, that is specifically why I only use 4-ply CDX or better now-a-days, although it does cost several dollars per sheet more than the 3-ply.
One time back around 1991, the lumber supply shipped out the wrong sheathing on a job we did and when I got there and noticed it, I made my crew remove the entire one side of the house that they installed and replace it with the proper product. This was when I was still experimenting with different decking products and we did not use 4-ply exclusively. If that were to happen currently, my crew would know immediately.
OSB, well…, you will get multiple opinions on that cr*p.
I usually find that if a contractor had to pay the exact same amount for the 2 different products, then they would purchase the CDX, but money talks so loudly when it affect the pocket book, that they will still proclaim it to be better. That goes against the actual manufacturers reports defining which product performs better and with less adverse affects. (I reference a report and study with conclusions between the 2 different products, done by Georgia Pacific, who manufactured both styles of products. They have since then, shifted their manufacturing of OSB to a Canadian division, to limit liability issues. The report was done while they still manufactured both products though, which I feel to be a very unbiased source, considering)
At least the guy and his crew that screwed up is local to your area, so you should have some leverage.
Check with your building code for deck sheathing “Minimum” requirements and see if 3-ply is even allowed. That will give you the best leverage on getting the job done the “Right Way”, like it should have been done in the first place.
If they specified 4-ply in their contract and stated that they only use that version, there should have been no reason that the wrong product got supplied.