You can use a 6 ohm resistor, but put it after the speaker, and wire it IN SERIES, not in parallel. It won't put out as much volume, so be ready for that. The resistor should have at least a 1 watt rating, more would be better if you plan to turn it up more than a quarter of the way. 6 ohm is a nonstandard value, so a 5 ohm would be OK.
If you can't find any resistors that small, you can make a resistor out of Nichrome wire.
Finding an 8 ohm speaker that would fit would be much better overall. You could probably run it without a resistor as long as you didn't turn the volume up past 3/4.
You could also find another 4 ohm speaker and run it IN SERIES with the first speaker. This would give you an effective value of 8 ohms.
There is a distinct reason why you don't want to run a 4 ohm speaker. As the resistance drops in a speaker, it gets harder and harder for the amplifier to drive it. An old radio will not have the power available to drive the 4 ohm speaker, so it will end up destroying the amplifier and most likely the speaker.
If anybody wants to know why this is, I'll explain the math formulas if you request it. Otherwise I won't bother.
