This is not a valid Processing version.
-Restarted / "reinstalled" Processing, startet it as admin in XP-compatibility mode
Why? The Arduino IDE and the Processing application are completely independent applications. The Arduino IDE is what you should be using to upload sketches to the Arduino.
-Arduino Mega 2560 connected on port 4
If the Arduino is really on COM port 4, the Arduino IDE wouldn't know/care that COM3 is in use.
What you need to do is start the Arduino IDE without the Arduino connected. Make a list of the COM ports that show up under the Tools + Serial port menu item.
Then, close the Arduino IDE, connect the Arduino, and restart the IDE. Check the list of COM ports again. The new one is the one that the Arduino is connected to.
No new ports means that the necessary driver is not installed.
A new port, that still is shown as in use, means that some other application is accessing that serial port. You need to figure out what that application is and not just close it. You must uninstall that application, burn all packaging and documentation for that application, and refuse to ever do business with the asswipes that supplied it.