The 'loths can serve a multitude of "roles" regardless of whether you're using the meta-setting elements of the Blood War, or the harcore trappings of Planescape. Even within their ranks you'll find different incarnations of just what Evil as a concept is. Their lowest are selfish and opportunistic, but as you go up their heirarchy towards what they would regard as more "pure" forms of evil, you'll find a paradoxical undercurrent of what amounts to quasi-religious zealotry in the service of universal misery from beings who yet care only about themselves. They're not evil trying to randomly destroy or control, they're an ancient and primordial evil that predates mortality but happily feeds upon it like a disease, and evil as the obliteration of hope.
I'm cynical on the matter since I suspect that 4e won't include them given the bullet in the head the guardinals received, but the 'loths would fit as servants of the so-called Primordials (and really that strikes me as being the same function that the baernaloths and other such hinted at beings already represented). Alternatively the 'loths could be strangers in the purest sense, beings of malignant mystery with their own unknown goals whose origins are an open question and their mercenary activities being much the same as now (and easily eating the ecological niche of the mercane as well). Maybe they themselves don't know their origin, or even what they are. They might be from another multiverse altogether; stranded, bitter, and ultimately willing to drag all beings down into a hell of their own creation. There's no reason to even define their history in a hard sense if you're trashing what already came before 4e, which I suspect will happen if they're used.
Of course, given the complete and utter silence on the DI's end regarding yugoloth proposals to Dragon, my hopes aren't up that they'll even be considered in 4e. Perhaps picked apart and shoehorned into the "role" of a 4e demon or devil rather than being reinvisioned as something distinct as they were previously, but if the former I'm probably not interested in them anymore.