Finding the Id of the puck programatically

I'm probably a bit late to this but...

In the API documentation there is a section on the Puck class (here: http://www.espruino.com/Reference#Puck) but it seems to be missing a lot of stuff - for instance there is nothing in there about Puck.connect?

Ahh - those are the functions that are available when running JavaScript on Puck.js itself.

Puck.connect is a command that you run in your web browser, which uses the Puck.js library. There's some information on it at http://www.espruino.com/Puck.js+Web+Bluetooth but there isn't actually a proper reference. I'll make a note to add one, but in the mean time if you look at the source there are some examples at the top, and if you scroll down to the bottom, the exported functions are there with explanation for each.

Now I would like to know its four-character Id.

The 4 character ID at the end is just the last 4 digits of the device's MAC Address.

You can get it on the Puck itself with the command NRF.getAddress().substr(-5).replace(":","") -
which takes the address in the form aa:bb:cc:dd:ee:ff, strips off the last
5 characters, and removes the colon.