Doing that won't affect the damping factor of the other three drivers. But perhaps you don't even have to add the resistor. Three nominally 8 ohm drivers in parallel with a nominally 4 ohm driver results in a nominal overall impedance of about 1.6 ohms. If you step that up 4x with the Zero, and probably also 3x or possibly even 2x, your amp should be happy (in terms of the load impedance it sees; I'm assuming the amp can supply sufficient power for your application).
Keep in mind, though, that without the resistor the 4 ohm driver would receive twice as much power as each of the others, while with a 4 ohm resistor the 4 ohm driver would receive half as much power as each of the others.
Regards,
-- Al
Keep in mind, though, that without the resistor the 4 ohm driver would receive twice as much power as each of the others, while with a 4 ohm resistor the 4 ohm driver would receive half as much power as each of the others.
Regards,
-- Al