To achieve impedance (ohmΩ) standardization, all speaker manufacturers now measure impedance (ohmΩ) at 400 Hz. This measurement gives you a correct A.C. impedance (ohmΩ) of a speaker. Therefore, at 400 Hz your 8Ω speaker is actually 8Ω (or close to 8 Ω). However, anywhere above or below 400 Hz the impedance will vary.
Measuring a speaker on an ohm meter will actually give you a D.C. resistance reading, not the A.C. impedance (ohmΩ) of the speaker. To measure A.C. impedance (ohmΩ) of a speaker you need an impedance bridge capable of measuring at 400 Hz. The D.C. resistance of a speaker is generally + 3Ω of the correct A.C. impedance (ohmΩ).
Parallel  
If both speakers are 4 Ω the nominal impedance is 2 Ω If both speakers are 8 Ω If both speakers are 16 Ω 

Series  
If both speakers are 4 Ω the nominal impedance is 8 Ω If both speakers are 8 Ω If both speakers are 16 Ω 

Parallel  
If all 4 speakers are 4 Ω the nominal impedance is 1 Ω If all 4 speakers are 8 Ω If all 4 speakers are 16 Ω 

Series Parallel  
If all 4 speakers are 4 Ω the nominal impedance is 4 Ω If all 4 speakers are 8 Ω If all 4 speakers are 16 Ω 
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