It will limit amplifier output characteristics in all cases you set, but if done to 8 ohms, your amp may half or even quarter the power to the 4 ohms while to 8 ohms it may have some voltage deficit and probably just half the power.
Last thing you want to do is to half power to your mains. your center channel will mostly work on mid-range and will have enough spl volume.
Last thing you want to do is to half power to your mains. your center channel will mostly work on mid-range and will have enough spl volume.