A ket design element of 4e is "The monster/NPC has what it needs to do what it has to do in the story". This is great if you have any smeggin' idea where the story is going. It's less useful if you're busily making it up as fast as you can. When that happens, having an NPC with 20-odd spells prepared means it can fulfill a whole LOT of different "roles". (Last night, the players were convinced someone in the local church was a cultist of Orcus. I hadn't wanted to go with the whole 'corrupt priest' routine, but they were wasting soooo much time trying to find one I eventually decided, fine, there is one. Naturally, the PCs put him in a bad spot. Based on my hastily deciding his level, he had Slay Living, which let him off the High Bishop when the PCs tried to force a confrontation, then he won initiative and Plane Shifted away. Next round, the bishop's guards rush in to find a dead bishop, two PCs, and no other suspects. To be continued!)
ANYway...
From what I've seen of 4e, people don't have that many different powers. It's believable for a 10th level 3e cleric to have those spells prepared; less so that a 4e NPC would, since those would likely be the only powers (or the rough equivalent) that they'd have, period. A "Gating Assassin" might, but that would mean I would have needed to plan for this in advance, and "plan in advance" isn't my strength. I build worlds, not scenarios. But getting back to my point (and yes, I have one...)
Could there be a ritual called "Grant Power"?
You perform the ritual, and you gain a spell/prayer/exploit which you can keep for (an hour? Until used?). This allows NPCs to have a few useful powers "stocked", without them being part of their basic template. Make it expensive enough, and it won't be exploited -- or, hell, just make it an NPC-only trick, though I tend not to like those. Make the cost something like 100 gold * power level ^ 2, and I don't think you'll end up with a lot of PCs using them, though it does give NPCs some unexpected tricks, and helps justify, even retroactively, an NPC having just the right ability when he needed it. ("Yeah, he, uh was prepared for that...")
ANYway...
From what I've seen of 4e, people don't have that many different powers. It's believable for a 10th level 3e cleric to have those spells prepared; less so that a 4e NPC would, since those would likely be the only powers (or the rough equivalent) that they'd have, period. A "Gating Assassin" might, but that would mean I would have needed to plan for this in advance, and "plan in advance" isn't my strength. I build worlds, not scenarios. But getting back to my point (and yes, I have one...)
Could there be a ritual called "Grant Power"?
You perform the ritual, and you gain a spell/prayer/exploit which you can keep for (an hour? Until used?). This allows NPCs to have a few useful powers "stocked", without them being part of their basic template. Make it expensive enough, and it won't be exploited -- or, hell, just make it an NPC-only trick, though I tend not to like those. Make the cost something like 100 gold * power level ^ 2, and I don't think you'll end up with a lot of PCs using them, though it does give NPCs some unexpected tricks, and helps justify, even retroactively, an NPC having just the right ability when he needed it. ("Yeah, he, uh was prepared for that...")