No idea why, but Wired sent me another YubiKey 4. Guessing it was a mistake. Lucky me! Being the honest sort of chap I am, I will ask them if they want it back but they'll probably say to keep it. Especially since they no longer are offering a subscription with a free YubiKey.
Note for those interested in just the magazine, they are offering $1 for 3 months right now for just digital version or $3.50 ($1 + $2.50 shipping and handling) for paper version.
They let me keep it, by the way but boy do they take a long time to respond to queries. So now I have my backup key that they always suggest you have in case you lose your primary.