I would LOVE to have a detailed answer, but sadly when things go in the wrong direction, buyers who got no clues about real 4-mmc, or they enjoyed 3-mmc thinking is 4-mmc, are now getting all the other dozens of analogues thinking they are just "better" or worst batches. As long as the big vendor whales offer the product, and their other product are legit, the "culture" of the real Meow Meow will be sadly gone..
It's just cheaper and hassle free for the big vendors to get from their wholesale reseller they trust whatever react positive to cathinones, and wholesale is just buying from other EU manufacturers but mostly from china the "codename" unknown random ones. That's the main reason: china can manufacture very cheap big quantities.
The "culture" of 4-mmc started to spread / raise again with 3-mmc, since it was the closest shot (even if still miles away), and this was a tasty occasion to "scam" a lot of returning real mephedrone lovers. Manufacturing of 4-mmc is made mosty (and only, if we talk about EU) in UK, where it never stopped existing, and Poland. NL/SWE and other east countries it's a coinflip about what you will get. But the lab drugs checking made in the last 2-3 months in EU talks clearly: not a single one sent to energy-control (Spain) or eve-rave (Switzerland) was 4-mmc, not even 3-mmc anymore.
UK is high risk country for shipping, same for Poland (for the shipper, not for receiving on the second one). So the real 4-mmc stays in the hands of the lucky ones who can get a reshipper in one of such countries.
And yes, *bump* still waiting for a real 4-mmc manufacturer EU-based who can supply 4-mmc - both small and big quantities.