Ok, then I will go ahead to buy a laser to use it as a light barrier, because the one I have at home shouldn't be damaged.
I could also create a voltage divider - calculated with this, I'd have: Vin=5,R1=220,R2=1K,Vout=4. That should be ok.
I already also heard, that the batteries are a bit more than 1.5V, maybe 1.6V or 1.7V. A new "1.5V"-battery I measured had a bit more than1.6Volts. That means that 1.6V*3=4.8V should be definitely still in the acceptable range for the laserpointer. Most of them are from China and hence don't have a manual with technical parameters.