Bob's basic statement is true - speaker impedance is more complex than just DC resistance - but you can get a fair idea regarding the basic impedance of the woofer with an ohm meter. What you cannot easily predict from DC resistance measurements is how or how much the impedance changes at higher frequencies. Ideally the designer kept the impedance relatively constant but there are no rules saying they have to and a lot of history suggesting the designer may not - in order to solve a particular problem.
The second statement that all amplifers will drive 4 ohm loads is too sweeping. A lot of the amplifiers in "receivers" couldnt handle the added amp heating generated by their class of operations and the increased current demand of 4 ohm speakers - some simply shut down if driven at all hard, others dealt with it thru other means like inserting a low value resistor in the output for 4 ohm loads - via a switch. Tube amps usually use output transformers and these should be impedance matched to the speaker - usually done by switches or taps on the output transformer. Better solid state amps certainly should handle a 4 ohm speaker just fine, but my guess is a lot of surround speakers are not driven by amps as good as used for the front.
