German Grand Prix circuits

[gallop]

Hi all

yet another challenging question. This time I'm looking for an answer
to this:

Name four motor racing circuits in Germany that have hosted Grand Prix.

Three are easy: Hockenheim, Nurburgring, Berlin (Avus). But there
must be a fourth, and that's the one I'm looking for. You could,
of course, argue that Nurburgring was in fact 2 circuits, and there
was a whole series of non-championship races at places like Leipzig, Rostock, Karl-Marx Stadt, Sachsenring, but I don' think these count.
I guess I'm after something in the time before 1950.

Any and all help would be much appreciated.

regards

Neil Gallop