[Absolute Speculation]
"Some will see mystery and some will see nothing. Some will see hope and others will see only despair and a multiverse that provides them no firm answer to the question, "If I have lived a good life, will the life after this one be any better?" In the absence of objective fact, only the so-called truth of gods and the self-delusional assertations of their true believers, everything is pitiless and gray, a mirror to the souls of mortals in which the only answer they find to their question is an echo. In the absence of reward for Good, and justice for Evil, there is despair, and where there is despair, we are there to feed and flourish." - Sarnatharius Ibn Sheolloth, 9th Prophet of the Screaming Gulf, advisor to his Faceless Majesty the Oinoloth of the Shadowfell, Mydianchlarus.
Frankly, if they're sticking the major fiend races in various scattered pockets of the planes rather than having any sort of unified Lower Planes based around alignment/ethos/physical manifestations of Evil, then the yugoloths fit the bill more for a variety of reasons. Unless the 'loths simply don't appear in 4e, or if they get totally rewritten in terms of their core substance rather than adapted and given a new spin in a new setting.
[/Absolute Speculation]