So running it at 4 ohms won't damage the speakers right? His main concern was frying his speakers in a couple years after running it at 4 ohms that long.
The 4 ohm impedance of the speakers, by itself, won't cause damage to the speakers. That is not an issue to worry about.
High sound pressure levels (SPL), or, in other words, "playing too loud" is where any speaker damage's root cause will lie.
The unit that is at risk of damage, due to the low impedance of the speakers, is the power amplifier (or power amplifier section of an integrated or receiver). If the Onkyo is specified to drive a 4 ohm load, then the assumption is it will not be damaged by such a load. Each amplifier will be somewhat different in this regard, so check that specification if you replace the power.
A multichannel amplifier should not experience any issues with certain channels driving one impedance and others driving another. No two different loudspeaker models will have the same impedance curve, so it's pretty much implied that this is a normal operating condition in any multichannel setup that does not consist of 5 or 7 identical speakers. Keep in mind the previous limitation about the amplifier's specification regarding it's ability to drive a 4-ohm load, and you should be good to go.
The one thing that might happen is there will be a mismatch in levels; some speakers may play louder than others. If you can adjust individual levels, you can solve that.
There are basically two situations that cause speakers to be damaged, but the root cause is identical ... heat. Impedance affects the power than can be delivered to the speaker, but it's an indirect specification that will lead you in the wrong direction. After all, "loud" is "loud".
An solid state amplifier* is capable, in theory, of delivering twice as much clean power into a 4 ohm load as an 8 ohm load (and 4x into 2 ohms, etc). Read on to learn how that affects performance or speaker damage, but the short answer is "it's a good thing".
Whether it actually delivers twice, or somewhere between 1x and 2x the power is dependent on how the unit was built and designed. The power supply has to be capable of supplying that power demanded, and the output stages (power transistors, basically) have to be able to handle the demand.
Since these are two areas that cost significantly more $$$ to build into the unit, not all amps will have the robust power supply, or the output devices rated that highly, to pull off 2x power in watts at half the standard 8 ohm impedances. After all, when we compare amps, it's power into 8 ohms that gets the large type in the brochure and the ads. Manufacturers want to sell product, and they know that the first step in a sale is getting your attention.
Back to damaging speakers with amplifier power. How do speakers get damaged?
The first, and the one most consumers worry about, is putting too much power into the voice coil by playing loudly with a powerful amplifier, and in the consumer's mind, this is situation of using an amp "with too much power for those speakers".
The second is heat damage caused by amplifier clipping, which is caused by using an amplifier of relatively low power. Again, in the ordinary consumer's mind, this is essentially not an issue, since the speakers "can handle 200 watts" or some such silly specification that is thrown about to differentiate one product from another, and in the end, to sell things.
The second condition is far more dangerous than the first, and in my experience the first is very rare to the point that people who do not do things like using dad's stereo at the wedding dance probably won't ever experience it in their lifetime.
Loudspeaker damage from amplifier power is almost always heat damage; the excess power applied to the drivers causing permanent damage of some sort (burn marks, coil heating to the point where it no longer stays tightly wound on the voice coil, the voice coil breaks, or expands and physically rubs against the gap, etc).
You can build speakers with more robust voice coils, or larger gaps to aid cooling, or other such things. Trouble is that is the path to poorer sound quality; the work of reproducing music via loudspeakers is the work of fine tolerances.
Clipping, at it's worst, consists of the equivalent of square waves being applied to the voice coil. There is virtually no time for the coil to cool between swings of the waveform; in essence, you've created a very crappy heater (crappy in that it won't heat you or the room much, but certainly will heat itself enough to self-destruct).
High power amplifiers that can operate cleanly at considerable output levels will still be producing waves that can be recognizable as music if you measure it; in other words mostly sine waves. A sine wave is "off" some of the time and "on" some of the time, with the majority spent somewhere between these two. There is considerable time when instead of being heated by the power supplied by the amplifier, there is an opportunity to cool as well.
Every single moment you play music with speakers, you are heating the voice coil. So, it's not that manufacturers don't take heat into consideration.
If you are playing at a level that would require 200 watts from the amplifier, a "250 watt" amplifier will be producing nice clean sine waves. A "25 watt" amplifier will be producing pretty much nothing but square waves. They both will be playing "loud"; there might be a quality difference in the sound, but the same speakers driven by the smaller amplifier won't be appreciably less "loud".
In other words, "loudness" and power being delivered are not so easily correlated by just using the gear.
However, the speakers driven by the smaller amp, driven into clipping, will be very much hotter at the voice coil, and very few loudspeakers are built to withstand acting as crappy heaters for a very long time. Maybe they can last a week, maybe a day, maybe five minutes, without heat damage. Those are just the details.
The cause, however, is using an amplifier of too little power. And if you are paying attention, you will realize that when a manufacturer states his speakers "can handle 200 watts" he's just throwing crap at you, because clearly we can create a system of 25 watts that will destroy them, and we can run them with a 250 watt unit that will treat them like family.
In some cases the manufacturer is trying to help you choose an appropriate amp for the speakers by specifying a "recommended amplifier power", which should give good performance. It's OK to go higher, though, when choosing the amp.
If the number is used as some kind of comparative performance specification, or if it's implied that it's a measure of quality, you are being misled.
Sales people who miss-apply that number throw the crap because they see consumers making buying decisions based on that worthless number. Being "right" doesn't put food on the table; whatever moves the stuff out the door does.
If you see something like the salesman suggests one model is "better" because "these handle 200 watts", or ... you hear this sometimes ... that the speakers "put out 200 watts", run, don't walk, outta there.
That's just gluing a bigger number on the speedo dial and calling it "high performance".
* Vacuum Tube power amplifiers, (and some solid state amplifiers, in particular older designs) use output transformers. Generally these do not deliver higher power into lower impedances; instead match the impedance of the speaker to the correct tap on the amp.
