Whether it's a 15-minute adventure day, or a 2-hour one (see my thoughts
here), or a 4-hour one even, the bottom line is that healing surges will run out and force the party to rest in a period of in-game time which I consider to be unreasonably short.
The problem isn't a mechanical one, for me. I think the
mechanics of the 4E system are lovely and elegant. On paper. They work great . . . if you're not trying to tell/co-create a
story that makes any kind of consistent sense.
So, my players want or need to rest after just a few hours in the dungeon, after only defeating 20-30% of the enemies within, perhaps. I know that if they proceed, they'll die. They know it too. So what do I do?
I can just let them take the 16-20 hours in-game and rest, unmolested and with no negative consequences, completely disregarding any kind of logic as well as ignoring the glaring inconsistency this presents in comparison with
any fantasy story that I'm aware of. That makes the
game work best, and a lot of DMs probably do it, and thus have no problem.
Or, I can apply some kind of logical ramifications of the PCs busting into some lair of villains, killing a couple dozen of them, leaving many others alive, and then deciding to hole up in or near the dungeon for a LOT of hours to sleep. Surely they'll be discovered, surely an alarm will be raised, no?
Do I send monsters to attack them in their camp? This would probably actually happen frequently given the situation, but if I do it, the party probably dies. And it feels like punishing them for just doing what the system forces them to. That's no fun for anyone.
Do I instead have the alerted foes rally and bolster their defenses, thus making the rest of the dungeon harder than it was meant to be? Again, punishing the PCs for playing the way they're supposed to.
Do I give the PCs a motivation for pressing onward by having some kind of time limit or urgent situation which will result in tragedy if they delay? This is probably going to get them killed, unless I just make the adventure really easy. And how fun is that?
Do I contrive every adventure setup to include just a perfect amount of encounters, just perfect places to rest without screwing themselves, just perfect layouts of location to be ideally convenient for an adventure using the 4th edition D&D ruleset? That's not only a lot of work for me, it's also very fake-feeling, and ties my hands creatively quite a lot.
What about published adventures? With the exception of the really short ones like Kobold Hall and the RPGA 4-hour modules, running any kind of published dungeon is going to be practically impossible without running afoul of this problem, changing the modules a lot, or contriving completely absurd rationales all the time.
"No, for some reason, all of the rest of the bandits in this bandit compound just never bother to go over to the area you cleared out, and don't notice the pile of bodies you left, or wonder where George is. You rest for 20 hours and come back to find everything as normal."
In 3rd edition, at least the PCs had craploads of wands and scrolls after about 4th or 5th level. Every Tom, Dick, and bugbear barbarian had a backpack full of
cure x wounds wands, so running out of healing was sort of a non-issue. There were problems with this, and actually I'm really glad that they did away with it in 4E. I think the game is measurably better without all of that.
But it leaves us with this hole now where the PCs just don't have the hit point/healing resources to get through a full day of adventuring, and there's absolutely no way within the system to circumvent that, not even at 30th level. All that one can do is either deliberately run the game in a stupid, immersion-breaking, story-killing way that everyone at the table can sense is a bit silly . . . OR change the rules themselves, which brings up the whole concern of
"Am I breaking the balance of the game's design with this change?"
I don't think it's insurmountable, I just wish that this issue had already been taken into account by the framers of the new rules.