Older receivers (G9000 series and back) were typically rated for minimum of 4 ohm load. This was done to accomodate two sets of 8 ohm speakers, as many models supported two, or even three, pairs of speakers. Two pairs at 8 ohms run concurrently presents a 4 ohm load to the amp, as the speaker outputs are run in parallel and this halves the impedance of two sets of speakers that are 8 ohm. Two pair of 16 ohm speakers (admittedly not a common configuration since the 60's) would present an 8 ohm load under similar circumstances.
Of course, if you have one pair of 4 ohm speakers, you can run only that one pair on an amp that is 4 ohm rated. Two pairs of 4 ohm speakers would present a 2 ohm load, and you will definitely smoke an amp with that. This is also the reason an amp that allows attachment of 3 pairs of speakers still only lets you run two pair at the same time, and not all 3 at once.
Many newer receivers and amps now run the speaker outputs in series, which sums, rather than halving, the impedance. This has the effect of reducing output power however as it doubles the impedance, assuming both pairs are the same impedance to begin with.