more often than not, you arent hitting what your aiming at becase of the sights... (ruling out you are not flinching, pulling off ect)
i am a decent shot, but for some reason when i first got my glock 30, i couldnt hold a decent group, so i swapped out the factroy plastic sights, with square profile adjustible sights,
this is based on personal preference, but i shoot (handguns) better with square sights that give a sharp picture, as opposed to sights with rounded corners, or blade type front sights.
i prefer sights with a little ammount of space between the sides of the front sight and rear sight notch, so i can tell if the left-to-right alignment is correct. you can be surprised just how accurate the human eye is to alignment...
also, because your px4 is a polymer frame, do you have problems with it getting slippery? i know if im shooting, sweat, water, rain, mud and gun-oil can make the grip very slippery and hard to hang on to...
remedy this with some black electric tape. on polymer frames it wount harm the grips. but be careful with wooden grips.
also, hold the sights the same way and try to break the shot on the same place each time...
just keep practicing, you WILL get better with time and expirence. it takes time to get used to a particular gun, and a handgun is the hardest weapon to master...