Install a terminal of your choice.
On a 4 you could run Xclock or Oclock (i forget which is which) on a custom xserver.
I have demonstrated this exact thing working on several threads. (clock wasn't fullscreen but meh)
Re PDF: Install the kindle 4 Liberator and see if that suits you.
(a virtual X server)
[root@kindle /proc]# Xvfb -fp /mnt/us/sh/fonts/X11/100dpi/ -noreset -nolisten tcp -screen 0 600x800x8
(a port into that to grab some graphics from, and to connect to from our PC to manipulate the X server in the absence of keys to get our best "setup" configured for keyless initialisation)
[root@kindle root]# x11vnc -noxdamage -noncache -forever -shared -nopw
(put onto screen for real)
[root@kindle root]# /mnt/us/kindlevncviewer/kindlevncviewer -config /mnt/us/kindlevncviewer/config.lua -encodings "copyrect raw" 127.0.0.1:1 &
(if the above fails OOTB on the four to actually show on screen run the command below in another terminal)
[root@kindle root]# while sleep 1; do eips ''; done
(let's have a window manager for the fullscreen handling - optional)
[root@kindle root]# blackbox &
[root@kindle root]#oclock (or xclock?) (plus some gumpf from the threads assuming you don't have your system setup exactly like mine - mainly -font 6x7 or w/e I forget exactly)
example here http://www.mobileread.com/forums/sho...0&postcount=70
That would do it.
Or you could use a terminal. or write a native app.. or a kindlet? maybe... or a ...
well you get the picture.