Everything is pretty stright-forward.

1. Go to http://code.google.com/p/linuxgenius/
2. Get the source for wizardpen. I used http://linuxgenius.googlecode.com/files/wizardpen-0.7.0-alpha2.tar.gz
   wget http://linuxgenius.googlecode.com/files/wizardpen-0.7.0-alpha2.tar.gz
3. Unpack.
   tar -xvf wizardpen-0.7.0-alpha2.tar.gz
4. Move into the unpacked dir:
   cd wizardpen-0.7.0-alpha2
5. Configure, make and install:
   sudo ./configure --with-xorg-module-dir=/usr/lib/xorg/modules && make && make install
6. Check for wizardpen.la and wizardpen.so (driver installed correctly):
   ls /usr/lib/xorg/modules/input/wizardpen.*
7. Connect your tablet (if not yet connected)
8. Create file /etc/hal/fdi/policy/99-wizardpen.fdi
9. I used the following data inside the file:
   <?xml version="1.0" encoding="ISO-8859-1" ?>
<deviceinfo version="0.2">
<device>
<!-- This MUST match with the name of your tablet -->
<match key="info.product" contains="UC-LOGIC Tablet WP5540U">
<merge key="input.x11_driver" type="string">wizardpen</merge>
<merge key="input.x11_options.SendCoreEvents" type="string">true</merge>
<merge key="input.x11_options.TopX" type="string">0</merge>
<merge key="input.x11_options.TopY" type="string">0</merge>
<merge key="input.x11_options.TopZ" type="string">40</merge>
<merge key="input.x11_identifier" type="string">stylus</merge>
<merge key="input.x11_options.BottomX" type="string">32739</merge>
<merge key="input.x11_options.BottomY" type="string">32745</merge>
<merge key="input.x11_options.BottomZ" type="string">1024</merge>
<merge key="input.x11_options.MaxX" type="string">32739</merge>
<merge key="input.x11_options.MaxY" type="string">32745</merge>
</match>
</device>
</deviceinfo>

* This source code was highlighted with Source Code Highlighter.
10. After all this You should reboot and start using your Genius G-Pen 4500.
11. FIXED: I still have one unsolved problem. The stylus starts working a bit far from the tablet. Can’t resolve this. Under Windows I found quite unique driver, which let me set sensitivity for it. FIX: Add TopZ and BottomZ values to the file