Could you elaborate on how this ruins the yubikey 4? As a security measure, the software on the YK4 cannot be updated. Therefore, the software in question could have essentially been implemented as hardware. It is hardwired and will never change. I can't find the reference, but since there's no software update functionality, I'm under the impression even Richard Stallman would be okay with it under these circumstances.
"As for microwave ovens and other appliances, if updating software is not a normal part of use of the device, then it is not a computer. In that case, I think the user need not take cognizance of whether the device contains a processor and software, or is built some other way. However, if it has an "update firmware" button, that means installing different software is a normal part of use, so it is a computer."
Edit: Found it. https://stallman.org/stallman-computing.html
"As for microwave ovens and other appliances, if updating software is not a normal part of use of the device, then it is not a computer. In that case, I think the user need not take cognizance of whether the device contains a processor and software, or is built some other way. However, if it has an "update firmware" button, that means installing different software is a normal part of use, so it is a computer."
Straight from the big GNU's mouth.