...when working with keyboard and DS4 as player 0 and 1 respectively rewire always thinks that the Keyboard and the DS4 are player 0
Keyboard controls are determined entirely by the keyboard maps you create and assign to Players in the editor. If you created only one keyboard map and assigned it to Player 0, then yes, the keyboard controls will only apply to Player 0. But you can just as easily create 8 different keyboard maps for 8 players, assigning one to each, and have all 8 players using the keyboard. The keyboard is a shared controller and is never assigned to any particular Player. The only thing that determines which Players have keyboard input is the keyboard maps.
Joysticks are a different story. A joystick must first be assigned to a Player before it can be used by that Player. By default, Rewired is set to auto-assign joysticks. When a joystick is attached to the system, it will be assigned to the next available Player who doesn't have a joystick already (specifics of how this works is determined by the settings you choose on the Settings page.) So, using the default settings, if you have 1 joystick attached, that joystick is assigned to Player 0. The next joystick you attach is assigned to Player 1, and so on.
Your difficulty came from the fact that you were assuming having a Keyboard Map assigned to Player 0 would then make Rewired skip Player 0 when assigning joysticks and give the first joystick to Player 1 instead. This isn't the way it works. Keyboard Maps do not in any way affect the assignment of joysticks. Joysticks will still be assigned on attach based on which Players don't have one assigned yet.
Multiple joysticks work just fine with these Rewired actions. I tested them and there are no issues.
Guys, maybe (MAYBE) you should read the Rewired manual again.
Very good advice. You really do need to understand the system to get the most out of it. I spent a ton of time writing very thorough documentation, so please make use of it.