I had good quality 4.0 Fibracell Premier Alto reeds from Thomann in the past with a hardness that seemed reasonable when comparing to 3.5, 3.0...
Around October 2025 I ordered another one since the old one broke. This 4.0 was extremely soft and felt like a 2.0. Ordered another one in November, exact same issue. Getting reimburesed by Thomann was tedious, but worked out in the end after multiple mails.
Interestingly, I had the same issue with another 4.0 I ordered from a online-store in Germany around the same time.
It seems there is a bad or mislabeled batch of 4.0 from Fibracell. Do not buy a 4.0 now and multiple shops seem affected.
I bought a 4.5 now that was a bit soft too, but overall fine and appropriate. Try those and sand them down.
Fibracell does not respond to customer complaints at all btw. I tried. Try reading other peoples experiences online. They do not seem to care at all. One can only hope they will listen to a big retailer like Thomann if enough people complain. If you got a soft 4.0, please tell Thomann. Right now it seems everyone buying a 4.0 will get disappointed and waste their money.