Then expand that somewhat further. In 4e terms, it sounds to me like you're considering only fights with an XP budget between level-1 and level+1, perhaps +/- 2 if you're feeling generous. Believe it or not, the 4e DMG--again, the book for the edition everyone decries as "too balanced"--explicitly tells you not to do this. For example, in the section Table Rules, it has a couple of paragraphs (p ) about "Metagame Thinking" (all emphasis in original):
Or these two paragraphs on page 30 (in the "Running the Game" chapter.) The first is part of a longer section about Character Death, and the second is part of the Fixing Your Mistakes section. All emphasis in original.
Or this section (p 56-57) from "Encounter Components," all emphasis in original.
...and here's the reference for page 104, which goes into much greater detail:
Notice, the description even allows for monsters more than eight levels above the party's level. It doesn't say, "never ever do this." It says, "Do this with care. The characters are very likely to die. The players should probably know what they're getting into, and have a clear shot at escape." So...do you believe 4e is a "balanced" game? If you do, then how can "balance" mean exclusively the "tiresome grind" that the book explicitly tells you not to use? The subsequent text basically says, "Make 10%-20% of your encounters Easy, 10%-20% of your encounters Hard, and the remaining 60%-80% Standard." A standard encounter is a solid challenge; if the party rolls very well and their enemies roll poorly, it might become a cakewalk, while if the reverse happens they may need to beat a hasty retreat. (I have done this multiple times in 4e play.) In 5e terms, "Hard" difficulty is probably closest to 4e "Standard," and "Deadly" is closest to 4e "Hard."
So, consider just calling it a "hard" encounter, without using the word "balance" in any form. What do you lose by changing that term? Do the descriptions I quoted above not permit for an encounter of this kind, even though 4e is the allegedly excessive "balance" edition?
Which is what the paragraphs above explicitly say, repeatedly. Mix up your encounters. Provide variation. Don't fall into a "stale" rhythm. Provide encounters where the party fights the same monster statblock, but seven levels later--so they can see how much they've grown. Throw unwinnable fights at them, if they know those fights are unwinnable or at least have a chance to survive if they're quick or clever or charismatic. Etc.
Again: Is any of that incompatible with the descriptions quoted above? (Do note, some of those solutions would involve re-factoring the challenge in a different way, e.g. if the party chooses to take over, trick, or intimidate the warren, that likely would be an impromptu skill challenge, with the DM making judgment calls about the difficulty of such a task.)
Because people talk a really big game about how 4e is this horrible, draconian nightmare of a system where you are forced to make everything in near-perfect lock-step with the characters. And then when you actually dig into their claims and show them the game text, every single thing they talk about is right there, explicit in the text, without need for interpretation.
So: Are the encounters you're talking about unbalanced? Or are they simply ones where you, the DM, know that they can't be solved by brute force? (or, in the case of the kobolds, that they could be trivially solved by brute force, but maybe the PCs are powerful enough now to find other, more interesting solutions.)
