For example, would it be more helpful to the network to setup one relay that had 4TB of bandwidth, or two relays that each had 2TB? Does the answer change depending on the amount of bandwidth each relay would have?
One relay is probably better, both for performance and anonymity (assuming the relays would run on the same machine).
Performance: There's no obvious performance gain by running two relays with half the bandwidth, since you should add the other relay to the MyFamily and thus only one relay would be used per circuit. Additionally, a second relay would add overhead and maintenance work.
Anonymity: By running more circuits over one relay, the individual circuits get a better anonymity as long as the relay isn't compromised (the same reason why being a relay improves the anonymity for your own browsing).
If you plan to run two relays in geographically seperate locations, that could be worth it, though.
One relay with 4TB, with the relay using up the 4TB quota as fast as possible.
Faster relays are more attractive to traffic, so the network is better off if you move that 4TB in a week then hibernate for 3 weeks than if you throttled down the bandwidth to keep the relay active all month.
Plus, a single relay is easier to administer.