Always go 4, that way when you upgrade your system you won't have to upgrade the cables as well - the price difference is not that much. (Get 12 or 13 gauge, such as from Audiopile.net)
The above point is valid, but the exact answer to your question is 2. To further complicate matters, it is very common to see 2 conductor cable with NL4 connectors, which is also fine. You will have success with 2-conductor and NL2, 2-conductor and NL4, or 4-conductor and NL4.
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