Sorry to have ignored you for a day!
For back ground music I would use at least six, otherwise you won't have even coverage.
I have found this thread that may be useful.
http://forums.audioholics.com/forums/showthread.php?t=38081&highlight=Speaker+impedance+matching
Anunaki made the point at the end of the thread that this type of system is best wired mono. I obviously agree as I made the same point.
Now you have two options if you want to use stereo, you can't balance the impedance without using eight speakers. The same will be true if you use mono and want to use both receiver channels.
If you use eight speakers you can select either stereo or mono, but using mono gives better results in these situations.
To do this connect the first two speaker in series. That means ONE positive of ONE speaker to ONE negative of the other. Do the same for the rear two on that side. Then connect the remaining positives and negatives together. The ones with no wires on yet. These last terminals will also be the connection to the amp. This is called series parallel connection. Do the same for the four on the other side.
Now if you want to wire for only mono and use only ONE set of speaker connections on your amp do this. Wire the two front left and right speakers in series, the middle ones in series and the rear ones as well. Then parallel the three sets of series connected speakers and connect to the output of one channel only. Leave your receiver set to mono.
I don't advise using just two speakers for your application. It will be above background near the fronts, and virtually imperceptible away from them. This is especially true as there is a noisy kitchen environment involved.
I hope this helps.