Hello all from Paris, France.
I've had this Stoeger xp4 from quite a while now.
the piston is NOT the same as HW 40 and Beeman P17.

It is precise, it shoots straight, same sights.
BUT definitely a lack of power.
bullets do not obliterate the paper target correctly...
To get correct round holes in the target you need to:
1) get more power from the piston : add a 2 mm metal round (why not a coin) and glue it wih epoxy 2 composants glue to the top of the piston.
2) you have to rework the barrel entry, so that all pellets would fire
3) you have to re-shroud the end of the barrel

in a few words :
better exterior design (Italian maybe)
low quality control on the barell finish
low power piston that needs 2 mm extra to work decently.
same truglo style sights (exactly)
it is around €100 in France, but if you add the time you have to spend on it to make it work well, maybe it's not worse to buy...

HW40 is €140 in France, it is ugly but works well out of the box, without rework...
I hope you'll pardon my poor English.
greetings from Montmartre, Paris.