Add Twiddler 4 config files and installation docs

README.org

@@ -204,3 +204,22 @@ generate lists of English words which gradually incorporated more letters.
 If you want to try out the GeekHand ChordMap, you can use the [[twiddler.cfg]] file
 here according to the instructions on the Twidder website. Be sure and backup
 your existing twidder.cfg file before you install another!
+
+** Installing on a Twiddler 4
+
+The [[twiddler.cfg]] file above is for the *Twiddler 3*.
+
+The *Twiddler 4* uses a different configuration format, so it needs its own
+files. The GeekHand layout for the Twiddler 4 is split across two files which
+together make up the single chord map:
+- [[1.cfg]]
+- [[2.cfg]]
+
+Both files are required: they are the two layers of one layout, not two
+alternatives, so you need to copy *both* of them over to your Twiddler 4. Follow
+the instructions on the [[https://twiddler.tekgear.com][Twiddler]] website for loading configuration files onto a
+Twiddler 4. As always, back up your existing configuration before installing
+another!
+
+These two files apply only to the Twiddler 4. If you have a Twiddler 3, use
+[[twiddler.cfg]] instead.