Inter-relation between the four brahmaviharas?

I read once, online, someone (perhaps in the Tibetan tradition if that matters) saying that the four brahmaviharas are inter-related -- in particular, that too much of one is counteracted by a bit of another. For example: - Feeling too sorry for someone (too much compassion/*karuna*) is counteracted by a bit of *mudita* (remembering the ways in which that person is virtuous or fortunate) - Similarly, too much of each of the other three is counter-balanced by a bit of each of the others I was reminded of it by [this answer](https://buddhism.stackexchange.com/a/24551/254) ("in good there is seed of bad"). I don't remember the explanation I read, though, and I wasn't able to find the reference again. Is that anything you've heard before? If you know the explanation but don't know a reference for it, that's OK too (I'd be happy to read just your explanation of it even without a reference).