Hi
I think you should build one more identical to what you have. That would be easier to set up. You can always add the fourth later. I have four myself and sometimes I turn off the last one and notice very little difference. I could easily have lived with three. Setting them up properly is very important. Pay attention to the time domain as well as the frequency domain. If done right, three should get you 95% there. Hope that helps.
I think you should build one more identical to what you have. That would be easier to set up. You can always add the fourth later. I have four myself and sometimes I turn off the last one and notice very little difference. I could easily have lived with three. Setting them up properly is very important. Pay attention to the time domain as well as the frequency domain. If done right, three should get you 95% there. Hope that helps.