Chess: How Can I Prevent An Enemy Knight From Advancing to Square E4 If I'm Black?

Incorrect. The knight on g8 needs only 2 moves to get to e4 (g8-f6-e4). The best way to keep it off the e4 square would be to move your knight on b1 to c3, followed by moving e2-e4. Look into the Alekhine's Defence.