Hi Markus,
I’m also busy with a 4 player card game and I’m trying to learn from your code. Thanks for sharing!
With respect to the neural network I’m following a different approach: As my network is memoryless  (everything comes in via observations) and the game itself is turn-based I use the very same network for all 4 players. So the learning rate is 4 times higher and there is no need to share the weights.