It is not a piezo speaker as with 8 ohms of impedance it can only be a dynamic speaker, that is simply a 8 ohm coil driven speaker. Piezo elements are high impedance devices. I can't make out the contact arrangement very well, but if there are four pins it's probably just for better mechanical stability for mounting to the PCB. Simply take your ohm meter and identify two terminals that read out a nominal 8 ohms and use them, and ignore the other two. Also there is no + and ground for a speaker as it's designed as a AC device so is polarity insensitive, there are just two signal terminals, and you are free to ground either one if required to be ground based.
Lefty