The impedance is determined by how you hook up the Shiva driver. To get a 4 ohm load with the Shiva, you have to connect the 2 voice coils in parallel, that will present the amp with a nominal 4 ohm load.
You don't necessarily have to tell the amp you are hooking up a 4 ohm load to it, it'll know by the extra current demanded by the driver.
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Depends on the normal stereo amp. The majority won't be rated for 4 ohm loads. A few of the higher end ones won't necessarily be rated for 4 ohm, but won't have a problem with it. Unless the amp is rated for handling a 4ohm load, I'd be careful.