Yes, I seen this too. But the fact that this receiver puts out the same amount of power into 4 ohms as it does 8 ohms tells me it has a very limited power supply. A 4 ohm speaker could suck the life out of this receiver in a hurry. Most modern amps can almost double their wattage into 4 ohms as opposed to their rating into 8. If it was me, I wouldn't trust a vintage receiver with a true 4 ohm speaker.
This is also why nearly all speakers were 8 ohms back in the day.
This is also why nearly all speakers were 8 ohms back in the day.