OHMs deal with electrical resistance. Resistance is the opposition of electron flow in a circuit. A very basic explaination is that the lower the OHM the less the resistance the more power/current it will draw.
The 4 ohms is the impedance of the speaker - but it varies vs different frequencies. You treat it like a resistor - but the impedance can be as low as 2 ohms or lower at certain frequencies. This is a taxing load on some amplifiers (Pioneer I believe have problem with this)
It usually is not too much of a issue unless you drive the speakers very loud and then the transistor can overheat and shut the system down.
Marantz is a good company - you might write them about your question - and also your speaker company to find the real impedance low.
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