TerraDave
5ever, or until 2024
Yes, in 4E I don't think gods actually grant "prayers", though the cleric still has to access "divine power". And of course, some of the main things cleric spells used to do are now done by ritual.
In the original Deities and Demigods, I think it went by spell level. So low level spells came from "faith alone", then from powerfull non-divine beings, then from full gods, with only the greatest gods granting 7th level spells, then the highest. (or at least that is how I remember it). This implies that uber-fiends can grant some, but not all spells.
One other thing to consider for 4E is that powerfull npcs don't have to have a class, they can still have powers based on whatever they happened to be. So through rituals and npc specific powers, your demon cultist can do pretty much whatever you want, even if you decide they aren't "clerics".
In the original Deities and Demigods, I think it went by spell level. So low level spells came from "faith alone", then from powerfull non-divine beings, then from full gods, with only the greatest gods granting 7th level spells, then the highest. (or at least that is how I remember it). This implies that uber-fiends can grant some, but not all spells.
One other thing to consider for 4E is that powerfull npcs don't have to have a class, they can still have powers based on whatever they happened to be. So through rituals and npc specific powers, your demon cultist can do pretty much whatever you want, even if you decide they aren't "clerics".