I think I made some progress. I would stay with the solution of switch off the power with a mosfet. But I found another way to check if the C4 is powered off. The former solution with a toggle bit lacks some seconds until the C4 is really powered down. Now I connect the 5V pin to the ATtiny. It changes to ~1.5V once it is shut down. The LED acts as indication and as a zener and so ensures that the voltage level is fine for the GPIO of the ATtiny.
Furthermore with the mosfet the C4 can be power cycled. For this I added the switch, a short press reboots the C4 (by the GpioShutdownService App), a long press of > 2s power cycles the C4. I looked to the voltage supplying the C4 to see if there are spikes when I do so, but no, no spikes. This is also only the last option, if driving the car and the C4 does not respond any more.
There is also a connection for a fan. To have the right time immediately after boot, there is also a DS3231 high precise RTC.
I will now strip a board, it takes some time...

- C4 Car Interface.png (234.61 KiB) Viewed 2455 times