Hi Natalia,
I didn't get the point that only four partner types are supported in system. I can see other partner types like WK(Plant), LI(Vendor), O (Org Unit), VS(Shipping Point) etc. are also supported in the system.
and you said: If you need a different partner-function you have to modify the system
I guess here you are talking about partner type not partner function as we can create whatever partner function we need.
Regards
Amit