Active or passive, it shouldn't matter unless you're using an existing design, which it sounds like you will. So then you're going to want the same driver the design uses. If the design uses the 4 ohm, get that.
If you're going to roll your own design, or have someone do the design, it doesn't matter. The 8ohm will be less senstive. That doesn't really affect the designing though, other than level adjustment. Pick the impedance that suits your use best.
Side note, some receivers are 4ohm stable but require them to be set to 4ohm mode. Not saying your receiver requires this, but some do and they suck in those modes. At least the few I've heard suck. I just left them in 8ohm mode and ran the risk of shutting down.