4 ohm vs 8 ohm

CFD244 said
You need an 8 ohm sinlge, or two 4 ohm in series

You can put a 10w (probably 5 watt would do) 4 ohm resister in series with the 4 ohm speaker.

It's not 100% the same as an 8 ohm speaker but it should protect the output transistor.