Either I am blind or I don't know how to use 4 controllers instead of 2 as the game suggests (and 2 keyboard players) for 4 local multiplayer. Could be a dos limitation I assume. Or do you know a solution to change the two keyboard controls to joystick (gamepad)

I'm not sure, but I don't think this is caused by DOSBox, but by the game itself. In fact, in the game's manual there is a screenshot showing 4 players controls: two are on keyboard, each of the other two are using a gamepad.

While I have never used them, I know some programs capable of converting gamepad's inputs to keyboard's inputs:
- AntiMicro (Free, OpenSource)
- JoyToKey (Shareware)
- Xpadder (Commercial)