1. Octavarium.
2. Six Degrees.
3. A Change of Seasons.
4. In the Presence of Enemies.
Don't consider TCoT an epic, but it'd come fourth, ahead of ItPoE, were I to. Truthfully, though, it's closer in length to A Nightmare to Remember than it is to A Change of Seasons, takes me on no more of a journey than a song like Blind Faith (which does take me on a magnificent journey! But not on the same level), and unlike all of the other four it's not split into distinct movements with unique titles.
If there's going to be a divide between "epic" and "regular song," you've got to be a little discriminating, I feel. The Count of Tuscany is their longest song in the regular bracket, and a good one too - I ranked it ahead of ItPoE, it ain't no slouch - but if we're going to make distinctions, they need to be distinctive.