Regardless of the levels each orc possesses, there are 90+ orcs in the citadel. To think that a group of 4th level PCs will be able to slaughter them all during one strike is, to me, a little crazy. There are too many of them on the upper levels to allow a stealthy slaying without the bodies being discovered soon after. When that happens, the entire upper portion of the citadel will be roused, and realistically, the PCs could (or perhaps should) be overwhelmed (mass grapple checks, aid another ---> doom).
I'm sending my PCs (they'll be around level 6 at this time) to Xul Jarak as a "special forces stealth unit," but only after a large force of armsmen from Daggerford (FR Age of Worms campaign) and a number of NPCs and party mentors assail the keep itself, drawing out most of the orcs on the ground floor AND the wyvern. When the alarm goes up, PCs will slip in (climbing the outer wall using obscuring mist and silence spells), slice a few throats, and make their way down to the lower levels before any orcs from above know they passed through.
I can't imagine doing it any other way. Certainly a party without any area-damage spells will get in over their heads when 30+ orcs (perhaps more given the situation) gang up on the PCs. Sounds like a TPK to me if not handled with care.
It is, of course, possible to slip in, get downstairs, and clear out those lower levels without orcs from above realizing (unless one orc slips away to rouse the citadel, which is a strong possibility unless PCs are very perceptive).