To be frank, you should not have thrown the group into such a series of encounters when using the 4E rules.
Even though WoTC abolished spell memorization, the healing surge mechanic even more firmly reinforces the X minute adventuring day than the previous editions.
Oh that's just wrong. 4e is the best edition yet for that kind of series of encounter. I just finished a 24 themed adventure where the heroes ran through 4 very tough fight in a row (Level +2, +2, +1 (with tactical advantage to the monsters) and +4 (with tactical advantage to the players)). Similar scenarios would have been impossible in 3e.
He said that the party has expanded a lot of ressources in a big fight, whatever that means. Presumably a lot of dailies. They still have their encounter powers, must still have healing surge and can still heal a bit during the fight (SW) and between them. They can try to press on, but they will have to be tactical and not slug it out blow for blow until they run out of surge. The OP said he has a few light skirmish in mind until the big fight. Well, that would be level 0 or even -1. PCs can eat those like candy, with no dalilies and very litlle healing. Make sure they all have an AP ready for the last fight by handing a milestone just before it and they have a shot.
If this was 3e, it would be over. You can't press on once the spellcaster have thrown the big guns in that edition. Claiming that 4e encourage the 5 minute workday more than 3e is an indefendable position.
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You should just go ahead and continue the adventure. If the heroes fail, try to hedge toward capture to avoid TPK. If it means they fail the mission due to time constraint, so be it. Heroes don't always have to win and a cruel defeat sets the stage for a triumphant return. More importantly, if they fail because of bad tactics, gently point it out and hopefully they will play smarter next time. That's what's happening to my crew.
The various published adventures have shown this much; faced with the same challenge, some groups of PCs have been TPKed and other have had an easy time winning. Think of Irontooth and so forth. There can be many reason for this (i.e. DM being unreasonable) but overall the main explanation is that some group of players are playing just plain smarter than others.
Letting the heroes fail when they were facing a level appropriate challenge is not a bad thing; it's a signal that maybe they should try differents approaches. If the DM coddle the players and fudge things in their favor even when they are using poor tactics, they have no reasons to change their ways since it will always be good enough.