You can parallel two speakers to cut impedance in half, or put them in series to double them. This assumes speaker impedance is equal; If not calculate with the following equations:

Series:

ZT = Z1 + Z2 + Z3...

Parallel:

ZT = 1 / (1/Z1 + 1/Z2 + 1/Z3...)

Where:

ZT is total impedance
Z1 is impedance of speaker 1, Z2 is impedance of speaker 2 and so on.

If you use four speakers, all the same impedance, connect two in series and then connect each series pair in parallel. The total impedance will be the same as a single unit. So if you connect four 8Ω speakers this way, the total impedance will be 8Ω.