4@4ohms each in series or parallel and the fading depending if you use the front channels or rear channels of the HU will be lost, meaning all 4 speakers will fade using the fader on the HU.
4@2ohms in series only and same for fader, it will be useless.
You can also just drive one speaker, if you bridge the amp, maybe a center channel speaker that can handle 700 watts or 900watts RMS@ 2ohms