On the wallwart it says 4.5V. I just measured 8.6V.
I didnt know this before. Does it mean that I can use this wallwart to power the Arduino (because 5V sources are not suitable)?
Good measurement. Unregulated wallwarts always read 4-5V higher than the rated value with
no loads on them.
This one is probably NOT a good choice to run the Arduino bd, however, because the voltage
will drop too low when you start connecting loads to the Arduino. Better selection is a 6V
unregulated wall wart, 500 mA or better. The actual Vin to the Arduino bd needs 1-2V overhead
to keep the voltage regulator operating properly.