I've done that some minutes ago, it was easy but a litle bit strange:

first of all start a root shell
then start knx2hd

then WAIT (min 5 minutes) .... depending on the hardware it takes some long minutes to start...

then the installation window opens. What I did:
First I clicked on topic 3 to create the desired partitions and then I started with topic 1
selecting the Knoppix-like installation. An other try to install the debian system was also successfull
but this kicks you back into the stone-age: you have to install everything on your own, no network
no openoffice, nothing....... its really annoying

Currently the installation was fine, but not everything is working well (e.g. openoffice does not start
as well as the samba server). I think I found an explanation for this:
In the press release they tell us that HD installation of 4.0 needs minimum 12 GB!!!!!!
I gave 10 Gb to the partition and I felt very generous....

I will start over again with 20 GB and see what happens.

Good luck
Rudolf