The 4E ruleset is not very well suited to do hard, simulationist battles of large armies.
However, it is well suited to do a narrativist, movie-like scene of war. I would run a battle this way:
First, I would describe the battlefield, the armies facing each other, the nervousness of the troops, the flags, the drums, the trumpets...
Then I would start a multi-part Skill Challenge / Delve mix:
Part one: The Big Heroes have to rally up the different sections of the army and do their best to organize them into a decent fighting force. Here, the PCs could try giving a pre-battle speech, or maybe they could try to give strategic instructions to specific platoons, or maybe they could try to give a last minute greasing up to the trebuchets. Complexity 1 (4 successes before 3 failures), Primary Skills: Diplomacy, History, Bluff. Secondary Skills: Thievery, Arcana
Success means that the player's army gets a bonus for the next part of the challenge, and less opponents face the party in the first encounter. Failure means the party has to face more opponents in the first encounter.
Part two: The party is at the front of the army, defending against the first wave of attackers (or at the front of the first charge, depending on the campaign!).
For this I would choose some terrain like a castle wall that has just been breached, or maybe a section of the ramparts where the group faces enemies climbing ladders.
The enemies in this encounter are 4-6 skirmishers and 4-8 minions, depending on how well they did on part one.
Narrate this as the "closeup combat" shots in movies like LotR or 300, where the camera is focusing on the actions of the heroes, while the rest of the battle is background noise and blur.
Part three: Back to the big picture and the sweeping vista of cgi soldiers! Create a mixed skill challenge, tailored for your party. For example, the Paladin, Warlord and Bard may have to shout orders and encouragement to the soldiers under their command (Diplomacy, History, Bluff), the Spellcasters could have to engage in magic duels with their enemy counterparts (Arcana, Religion), The Fighter, Warden or Barbarian could engage in awesome feats of strength and brawn (Athletics, Endurance), inspiring their allies and making enemies fear them.
This time, make it a level 4 complexity (10 combined successes before 3 failures). If the party succeeded in part one, remove one success for each success gained in the previous challenge. Success removes opponents from the next combat, failure means each member of the party loses one healing surge (which can be described as the effects of fatigue, wounds or simply lack of morale).
Part four: Again, pick a combat encounter, this time against the elite of the enemy army. For example, a couple of brutes, mixed up with some artillery or lurkers, plus 3-6 minions depending on the success of part 3. The terrain could be the same of part 2, or it could be something else.
Again, explain to the players that the PCs are surrounded by the chaos of battle and that they are just fighting the poor sods right in front of them, out of the hundreds of enemy soldiers.
For Part five, the skill challenge could be something else. For example, the heroes have spotted the Big Bad Guy some 100 feet away. And he's about to... dunno, do something bad, like kill the good King, or complete a foul ritual, or... something!
The party has to make their way through the waves of mooks and reach the Bad Guy.
Complexity 1, the skills could be Endurance, Athletics and Acrobatics (for the melee guys) and Arcana, Insight and Religion (for the spellcasters)
Success means the final encounter has less opponents. Each failure removes a healing surge.
Part six is the climatic battle between our heroes and the Big Bad guy and his bodyguard!