I'm currently running a 4e sanbox game and there's really no problem at all. Mostly due to the fact that I have the benefit and pleasure to have very good players who can (and do) play they characters well and play them as characters, not a puppets.
The characters understand that their proffesion is a dangerous one and that they should conserve their resources in case a bigger danger appears. Not once they took the "easy" way out by blowing all dailies, because they could have regret it later. (Granted, not every DM have are lucky enough to have players like that)
If they venture somewhere where danger is higher than usually / deadly to them (and they usually know it beforehand, because they are used to gather every information available about almost every place they are about to visit, their survival instinct is really high*), they prepare contingency plans for quick retreats (or let the smart characters figure it out and follow the instructions). Those could consist of pre-planned escape routes, magic items used to temporary boost speed or traps laid beforehand should they find themselves pursued. Regardless of their classes, somehow every one of them finds a way to contribute in situations like that and I support them in this in every way I can.
*this is not a natural approach to the game by most players as far as I can tell, and it didn't fit for me well at first, but my players argued that there is only 2 ways how an adventuring character can survive to higher levels. Luck or preparation. And you can't depend on luck all the time. After a short discussion I had to agree with them
Unbalanced encounters them selves could be bad / unfun / whatelse, but they could also be made entertaing and/ or memorable. These are only a few examples of many.
Low level encounters in "unusual" circumstances
- villagers attacked by goblins
It's not hard for level 10 adventurers to kill 8 level 1 ro 3 goblins. However, it gets challenging when you try to lower the "civilian" causalities to zero. You need to identify which villagers are in immediate danger, prevent goblins to reach them (and not every time it is doable by killing said goblin), notice that there are 2 more goblins with torches preparing some arsony, a helpless child on a carriage that's already aflame, and a maiden hanging from the foor of a nearby barn by 1 hand with another goblin stopmping on it. Bonus: saving the day is more heroic than killing a beholder, even though the goblins were low level "losers". (There were some really creative moments of unusual farm equipment and adventuring gear usage involved that made me do the "jaw drop". Twice.

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- "bandits" (in this case more like local teenagers bored and "entertaining" themselves, lead by local baron's younger son) attacking a traveling merchant (in this case an alchemist too)
So, protect the merchant, protect the wares? Upon recognising the masked attackers as teenagers, decide to make the final blows non lethal? It's not that simple, of course. They were after the merchandise for a specific purpose and started to "test" in in the middle of fight! So there you are, saving the day again by pacifying the halucinating and berserking ones, while trying to revive and/or heal the poisoned and heavily drugged ones. Of course, some guards sent by the baron (because he susspected his son is up to something and sent them after him) arrive shortly and here we go again, baron's son is laying on the ground with some "suspicious" characters around. Diplomacy in the middle of the fray? Yes please!

Did I mentioned the contents of some of the broken merchandise got now mixed a bit and are about to explode? The alchemist should be pursued to find a solution to neutralise it quickly (or some of the heroes can, if they understand alchemy, of course).
- rampaging mutated ogre
Oh yes, our heroes quickly realised this one is too tough for them (as they suspected), so they just retreated to a nearby walled outpost, pursued by the ogre, only let it be killed by 40 or 50 archers, safely from the walls.
- a very old dragon resides in nearby gold mine, preventing the mining to continue. Rich reward is offered for its defeat.
This, of course, lures adventuring parties from across the country. Our group is no exception, but they suspect (yet again) this might be slightly out of their league. So, they decided to wait for other parties to show up and devise a sinister plan: One group approaches and challenges the dragon. Immediately, another one does the same. They start argue which one of those will have the opportunity to be the heroes here (while amusing the dragon who haven't had such a good entertainment for years). Now, 3rd group enters, joining the debate (the dragon still don't suspect anything fishy going on, due to continual successes in the skill challenge to keep him off track), finally escalating to asking the dragon a permission to battle each other and the winner would challenge him! (Now, which dragon - still without suspicion - wouln't agree, right?) Of course, all 3 cought him completly off-guarg and dealt with him quickly. While frontal assaoult by all 3 would be wellcomed by a dragon breath resulting in multiple causalities.