I have a receiver that's rated at only eight ohms to drive 8-ohm loads and I put a 4-ohm speaker, what's the consequence? Well, these days you would hope that well-designed electronics will never actually self-destruct so that you can damage them. Most receivers and power amplifiers have protection circuitry that if it's unhappy with the current that it's being asked to deliver it to the speaker, that it will cut out or it'll shut down or something will intervene to prevent any damage. Now, I'm not saying that that that's 100% the case. I'm sure there's some equipment out there that you can actually blow up or damage parts in if you drive them with the wrong load, like a 4-ohm speaker if it's not rated for it. But I would hope that they're few and far between.
More likely, other than the protection circuit kicking in, is if you're listening to a movie and you've now got a pair of 4-ohm front speakers, the front left and right that are doing a lot of the work, and that receiver is only rated for eight ohms and you've got the movie cranked up and you watch it, well, over time the power that the amplifiers are going to be asked to deliver, it's going to vary, right? Depending on the content in the film. But the thing is, is that if it has a lot of very loud content and you're drawing a lot of power, and that movie may last an hour and a half or two, is that what might happen if the amplifier or receiver doesn't actually shut down or go into a protection mode, is that it may overheat.
And again, if it overheats or it gets to the point of being uncomfortable and not safe for the output devices in the amplifier, most well-designed electronics should just shut off at that point. You might get a flashing light on the front or something that your manual tells you that there's some thermal protection that's kicked in.
