Our Z4 does have 6 speakers.
The 4 mid-range speakers (5 if you have upgraded audio) are all the same part number. Might be easier and even cheaper to buy the BMW parts.
I googled the part numbers to get ballpark ideas on price.
S677A Top HiFi 6513 9141501 $73.55 (DDM Tuning) $90.50 BMW of South Atlanta.
S676A HiFi 6513 9240641 (from 01/2012) 32,91€ (online-teile) older cars have a different part 65139133410 $58.60 BMW of South Atlanta
base audio 6513 9133409 $55.63/$40.06 (DDM Tuning)
The speakers are probably made by D&M PSS. D&M is a Japanese holding company that bought Phillips' car speaker business.
http://www.dmpss.com/
These speakers are also used in the 5 and 7 series
Here is the diagram for the rear.
Only top hi-fi has the woofer. Looks like it fits into a cutout behind the seat.
Top hi-fi also gets a center tweeter. You can see the cutout for it in my photo above.