Connect lower power speaker to audio amplifier
You don't need a speaker at all.  Get a 4 &Omega; power resistor, or parallel a bunch of higher resistance resistors to get the equivalent of 4 &Omega; at the required power handling capability.  For example, twentyfive 100 &Omega; resistor in parallel will give you 4 &Omega; at 25 times the power rating of the individual resistors.