voltage makes no difference. The 8 vs 4 ohms is a reference to your speaker's impedance. You can tell by looking at your speaker's impedance spec. Typically they're rated at 8 ohms, often 4, and it's not uncommon for them to be rated 6 or some other value. Keep in mind this is a minimal rating and the true impedance varies with frequency.
In any event, it's not something you need to worry about.
In any event, it's not something you need to worry about.