4 ohms vs. 8 ohms

I am expecting a Jolida 502 BRC. It has outputs for 4 or 8 ohms speakers. I plan to use my Usher X719 withit, which are 8 ohms. I also plan to run a powered sub (ML), I dont know the ohms. How will it affect the amp? Do I use the 4 or 8 ohm tap? Thanks