Amphimir Míriel
Explorer
A battle in 4e is probably best treated as a mix of combat and roleplaying and/or skill challlenges. Normal combat for those times when the PCs are in the thick of the fight, face-to-face with enemies. Roleplaying or skill challenges for things like strategic insights, tactical advantage, rallying your own side's forces and demoralising the enemy.
As Korgoth says, you wouldn't want it to be one long combat lasting dozens of rounds, that doesn't work. Instead there should be multiple combats of typical length with short rests in between. I understand that this is actually quite realistic for fighting in plate armour. The wearer would get overheated after a few minutes and have to retire.
Another thought -- during the combat encounters, treat parts of the battlefield as a hazard, with arrows or magical blasts randomly coming in from afar (akin to artillery in a WWII movie -- have the "shells" falling between the PCs and some kind of objective).
Patches of trampled mud-and-blood can form patches of difficult terrain, and you can have wrecked defensive works (hedgehogs, berms, ditches, & the like) as complications, too. Maybe toss in an abandoned or half-wrecked siege engine that creative players (or villains!) can exploit.
(Of course, that's more "how to run a mass battle with 4e" then "are armies valid".)
Completely agree with you guys. Maybe tomorrow I'll post a revised version of the "skill challenge / combat encounter mix" in post # 19... right now, I'm really tired and need to hit bed...
In the meantime, do you guys want to give it a try and post something similar? Maybe we can get a good consensus of how to run an epic-scope battle using 4E rules...