Pyrrho would you mind on expanding on how to know which speakers would benefit from a separate amplifier? I know the 4 ohm is part of the picture.
Low impedances are a big part of the picture. But another factor is sensitivity. Some speakers require more power than others for the same volume. That also brings us to the space in which the gear will be located, and just how loud you want to play it, which are other factors in how much power you need.
If the impedance is not an issue with the speakers you select, you should try them without a separate power amp. If the receiver gets too hot running them, then you may wish to get a power amp. If the receiver seems to distort at high volume, you may wish to get a power amp (though it may be difficult for you to distinguish between the amplifier distorting and the speaker distorting, and the added power amp will only help if it was the amplifier that was distorting, not the speaker; either way, though, if you are hearing distortion, you should turn down the volume). And obviously, if the receiver's protection circuitry causes it to shut down after extended loud playback, you need more power for such volumes and should get a power amp if you are going to listen that way.
But going back to the earlier point for a moment: I used to run a receiver that retailed for about $600 with speakers that retailed for well over $6k. It sounded great (as it should with such speakers). I replaced the receiver with one that retailed for about $1700, because I wanted more (and newer) features. It also happens to be able to put out about twice the power of the older, less expensive model. But in my case, the old receiver was capable of driving my speakers to levels that I found painful with crystal clarity, so the extra power was useless in my case. Unless I engage a feature that affects the sound, it sounds the same as before. So, if I had purchased the more powerful receiver for its power and not its features, it would have been a total waste of money for me. This is the sort of situation many people are in with their receivers; they just don't need any more power, so spending money on it is just a total waste of money. Of course, for those who need it, it isn't a waste of money.
I certainly would not try running my 3 ohm Apogee speakers (that I use in my main 2 channel system) to high levels with my surround receiver. But my home theater speakers are an honest 8 ohms, of normal sensitivity, so nothing special is needed to drive them. And so I do not bother with a separate amplifier for them, as I do not wish to waste my money on something that will not improve the sound. With my main 2 channel system, I use a separate power amplifier, because of the 3 ohm speakers in that system.
One other thing: It takes a doubling of power to get just a 3dB increase in volume (which also means a quadrupling of power for just 6dB, octupling [8x] for 9dB, etc), so if your home theater receiver is not powerful enough, you should be looking at very powerful amplifiers in order to make it worthwhile getting one.
