🚀 go-pugleaf

putting a passive component in line with your 4 ohm speaker will match
your amplifier but will absorb half the power output.  you want something
that delivers all the power to the speakers but presents an 8 ohm load
to the amp.  most common way to do this is to put a matching transformer
between the two.  they would either be rated as 4ohm to 8ohm or in terms
of voltage ratio (e.g., 140v:100v).  

but in answer to your original question, yes a 4 ohm resistor would 
protect your amp.

d