Turn based systems suffer heavily from passivity. Especially in a 4 or more player game, having to wait for 3 other players to make their moves is brutal.
You should allow players to do something while waiting for the opponent to finish his turn... like managing formations or preparing spells or building a town, etc.
Basicly, make players alternate between combat-turns and management/building turns.
While player A moves and orders his army, player B can give build orders, manage his town, etc. - then in the next turn, the roles are reversed.
That way, no player is ever passive and the game stays interesting. The only issue you will have is how to balance the first-move advantage: One of the players will start the game with a construction turn, while the other starts with a movement/army turn. To balance this, you could give the player who starts with a construction turn a slight movement advantage in his first army turn and give the player who starts with a movement turn a small construction speed bonus in his first building turn.
Creeps, treasures and random events around the map (Heroes of Might and Magic style) that are not based on opponent activity can greatly enhance the gameplay with this idea, as then the army turns don't feel useless when you still need to build up your army.