Adrian,
Those old stereo receivers had excellent power supplies and all but the very crappiest will usually drive 4-ohm loads. The power supply and big fat heat sinks only had to deal with two amplifier channels rather than five or seven as in today's AV receivers. So current flow and cooling weren't the problem they present in many current 7.1 receivers when they're connected to low impedances.
The only exceptions I recall were some of the tweaky British brands sold with plenty of smoke and mirrors (including notorious brand that refused to divulge rated power output and other specs). On the test bench, some of those didn't meet spec with one channel driven into an 8-ohm load, let alone 4 ohms.
Cheers,
Alan
