All VZ's are Euro 4 compliant from factory. The cats have more heavy metals in them to reduce emissions further than a Euro 3 cat. It is perfectly legal to fit a Euro 4 cat on a Euro 3 car, but not vise versa. Not saying it can't be done, just not legally. The cats will only affect emissions, putting a Euro 3 cat on a VZ won't make it run any different, you will only fail an emissions test. Sometimes in the SV6, putting high flow pre Euro 4 cats on will result in a CEL because the post cat sensors will read as if the cats are not working.
There aren't many manufacturers doing Euro 3 and 4 high flow cats. There are NO legal 100 cell cats.
To answer your question xSV666x, people don't fit Euro 4 cats to the VZ because they are expensive. Usually about $100 a cat dearer than a Euro 3 cat. Exhaust shops would rather fit the cheaper Euro 3 cat than risk losing the job over price if quoting on a Euro 4 cat. Some won't compromise, but most do. At the end of the day, it is up to the customer to be informed, and if they knowingly fit a non compliant cat, it is on them, but the shop may be liable as well if they did the fitting.