I've just bought myself a Yubikey 4 to experiment with.
The U2F and OTP features are of some interest, but the main thing I bought it for was PGP via GnuPG. I was disappointed to discover that it works (at least as far as gpg --card-status showing the device) on current Ubuntu (15.10) and even on Windows (!), but not Debian stable (Jessie). Still, this is quite a new device...
In every case on Debian/Ubuntu, you need to apt-get install pcscd
For the non-internet-connected machine on which I generate my master PGP key and do key signing, I can just use the Ubuntu live CD, but since my day-to-day laptop and desktop are Debian, this does need to work on Debian stable for it to be of much use to me. A bit of digging revealed this handy matrix of devices, and the knowledge that support for the Yubikey 4 was added to libccid in 1.20. Meanwhile, jessie contains 1.4.18-1. Happily, a bit more digging revealed that retrieving and using the testing package's version of /etc/libccid_Info.plist was enough to make it all start working:
[david@jade:~]$ gpg --card-status (28/11 20:20) gpg: detected reader `Yubico Yubikey 4 OTP+U2F+CCID 00 00' Application ID ...: XXX Version ..........: 2.1 Manufacturer .....: unknown Serial number ....: XXX Name of cardholder: David North Language prefs ...: en Sex ..............: male URL of public key : [not set] Login data .......: david Private DO 1 .....: [not set] Private DO 2 .....: [not set] Signature PIN ....: not forced Key attributes ...: 2048R 2048R 2048R Max. PIN lengths .: 127 127 127 PIN retry counter : 3 0 3 Signature counter : 0 Signature key ....: [none] Encryption key....: [none] Authentication key: [none] General key info..: [none]
I've raised a bug asking if the new config can be backported wholesale to stable, but meanwhile, you can copy the file around to make it work.
For U2F to work in Google Chrome, I needed to fiddle with udev as per the suggestions you can find if you Google about a bit. The OTP support works straight away, of course, as it's done by making the device appear as a USB keyboard.