Well, there is one Hercules Mapping Pack which includes mapping for the Dj4set, but that does not fix the problem.
Today I tried something different and installed the included Virtual Dj 7 LE 4Set software on my other pc, as it is set up specifically for the Dj4set controller. At first I had no mapping options at all (as it is the LE-Version). Then I activated the 20-Days Pro Trial Version and got all the options. But still the same problem.
I also noticed that the option in the Hercules Tray Agent switches back to 4-Deck mode from time to time.
Is there nobody with a 4set controller who uses the advanced 2-Deck mode?
