You won't hurt anything by trying them.
A tube amp can even stand a shorted output.
The amp will put out less gain with a 4 ohm load.
Also the amp will clip quicker with a 4 ohm load.
There is also more distortion with a 4 ohm load.
If the speakers are more efficient then alls fine.
You might look into replacing the Output Transformers
for long term enjoyment with lower distortion.
DanL
