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

Hi Al,
Thanks for the response. Now I'm thinking, since the zero has several input wires, maybe I could use the 3X wire on the three 8 ohm drive and then connect the 4 ohm drive by itself on the 2x wire. That way all of them will practically be 8 ohm. Would that work or will that just burn the amp? Thanks.
Thanks, Ryan. 
As I interpret the information provided at zeroimpedance.com, if the Zero is being used to increase the load impedance seen by the amplifier (as opposed to decreasing it) there is one and only one pair of output wires on the Zero that can be connected to the speaker drivers. The choice of 2x, 3x, or 4x impedance multiplication is made based on which pair of input wires on the Zero is connected to the amplifier. So it appears that you would not be able to connect the 4 ohm driver (plus any resistor you may place in series with it) to the Zero any differently than the 8 ohm drivers.
Regards,  Al

I see, I wonder whether it can be wired backwards. I'll test them tomorrow when I get them thanks 
No, you definitely don't want to wire them backwards. That will LOWER the impedance seen by the amp, by 2x or 3x or 4x depending on the connections. So the 1.6 ohm combination of the four drivers would be seen by the amp as well under an ohm. See this paragraph in their FAQ. Good luck. Regards,  Al 
I see. Thanks. I think the increased damping factor as a result of the zeros could help. I'll add the resistor to see how it goes thanks! 
Hi Al,
Thanks for all your inputs and help. The zeros are installed yesterday and they are great. To take care of the 8 ohm vs 4 ohm problem, I think I've found the ultimate solution. The 3 ohm resistor would reduce 50 watts of power (directly only to this speaker) to reduce to about 16 watts, which is not what I want. If all the other 8 ohm drives receives 25 watts, I want this 4 ohm to receive 25 watts also. Therefore, I will install a 1.5 ohm resistor which increases the power transfer to about 2527.5 watt given the error tolerance of 5% on the resistor, which is reasonably close.
The new damping factor for this drive will be 2.5 and with the zero's 4x setting, this damping factor should go up to 10 or so, which is again, reasonably acceptable.
Is my analysis correct?
Thanks, Ryan 
Hi Ryan,
Glad the Zeros are working out well. I did some calculations and I agree with most of your analysis about the 1.5 ohm resistor. I calculate that if 25 watts is going into each of the 8 ohm drivers, 26.45 watts will go into the 4 ohm driver, if it has a 1.5 ohm resistor in series with it. 26.45 watts vs. 25 watts is a difference of about 0.24 db, essentially negligible.
This is all an approximation, of course, because it assumes that the driver impedances are accurate and are purely resistive. But it all seems to add up to a reasonable approach.
The damping factor seen by the 4 ohm driver, though, will not be multiplied by the impedance transformation ratio of the Zero. It will be approximately 4/1.5 = 2.67. The damping factor of the amplifier, as seen at the secondary (output) side of the Zero, will correspond to the amplifier's damping factor multiplied by the 4x impedance transformation ratio, which (especially if you are using a solid state amp) will almost certainly be high enough to be neglected relative to the damping limitations imposed by the resistor. But since the resistor is on the output side of the Zero, the driver will "see" its resistance directly, not through the Zero's transformation ratio.
Putting it another way, damping factor equals speaker impedance divided by the output impedance of the source providing the signal to it. The output impedance seen "looking back" into the output side of the Zero will be the amplifier's output impedance divided by 4, which except for a few unusual cases mainly involving tube amps will be very small in comparison with the 1.5 ohm impedance of the resistor. Therefore the impedance seen looking back from the driver will be 1.5 ohms in series with (i.e., added to) a very small impedance, resulting in a damping factor very close to 4/1.5.
In saying this, I'm also neglecting whatever damping effects may result from interactions between the drivers themselves. Again, those should be negligible given that their impedances are shunted by the extremely low impedance that will exist at the output of the Zero.
Regards,  Al

Hi Al,
Sounds great. I really appreciate all the help this time (and last time too!). Now I feel much more comfortable ordering a pair of the 1.5 ohm resistors.
Thanks again, Ryan Huang. 
. Problem (A). I was inquiring about adding a resistor between the amplifier and a DIY speaker (1.8 ohm) and learned that only the "Zero Former" by Paul Speltz could solve the problem. Paul was nice and explained in layman's term to me why using a resistor could not work because it basically "kills" the damping factor.
Problem (A) solved by Paul Speltz using his Zero autotransformer. >>>>>>>>>>>>>>>>>>>>>> Problem (B). Now I got a pair of zero's coming, I got another problem. 3 out of 4 drives are 8 ohm and one is 4 ohm. Will adding a resistor between just this drive and the binding post mess up the damping factor of the whole system, or will it only affect the damping factor of this one drive?
Problem (B)? Angelgz2, did you discuss with Paul Speltz how to solve this problem using his Zero transformer? >>>>>>>>>>>>> IF all 4 drivers were 8 ohms the easiest and cheapest way to connect them to the amp would be to wire them in a series / parallel configuration. * First two 8 ohm speakers wired in series with one another. 8 ohm + 8 ohm = 16 ohms. * Second pair of 8 ohm drivers wired in series with one another, = 16 ohms. * Then parallel the two 16 ohm loads together which = 8 ohms. Connect the 8 ohm load to the amp. I doubt you can return the Zero autotransformers for a refund. So you may be able to use them on the 4 ohm drivers. The transformer would be wired in series with one of the 8 ohm speakers. The 4 ohm speaker would connect to the output of the transformer. If you can return the Zero transformers take part of the savings and buy a couple new 8 ohm drivers to replace the 4 ohm ones. 
Clever thoughts, Jim (Jea48).
However, as you'll realize placing drivers or speakers in series will not produce good results unless the impedances of the drivers or speakers vary as a function of frequency in a similar manner. Which ideally means that they should be identical models. Otherwise the voltages applied to the series combination will divide up between the two drivers or speakers differently at different frequencies.
For that reason I suspect that placing an 8 ohm driver in series with a 4 ohm driver boosted to 8 ohms with a Zero is unlikely to produce good results. Also, we don't know how similar the three 8 ohm drivers are to each other.
Best regards,  Al

Hi Al, At this point we do not know how Angelgz2 is using the 4 drivers. Are they mounted in a common speaker box enclosure or are they spread around a room in the ceiling or in the ceilings of several rooms? I checked with a good friend of mine that owns a B&M audio/video dealership. IF Angelgz2 is using the 4 drivers in the ceiling of a room or rooms the dealer recommends the Niles SS4 speaker switcher unit. Another option is the SVC4 which allows individual speaker volume control. . Jim 