We just purchased a batch of Yubikey 4s and I am unable to get mine to work with Duo. I’m following https://duo.com/docs/yubikey. It works on the Yubico website if I upload the config to Yubico. I’ve regenerated a half-dozen times or more with no luck.
I even tried configuring as 6-digit HOTP and it didn’t work. In 6-digit HOTP mode I noticed it emits lots more than 6 digits (I believe it is sending the public and/or private identifier before the 6-digit code). I had it put three codes into my text editor, stripped all but the last 6 digits from them and successfully resynced the token, so it looks like it works for anything but logging in. (I tried the 6 digit trick for logging in, too - no luck).
I can’t tell if the Yubikey OTP mode generates too many characters - I don’t know what Duo is expecting.