So, what kind of guidelines do people use when building NPCs and enemies with class levels? My players are going into a new dungeon that is going to be filled with enemies and NPCs with class levels, and I need to figure out how to balance it for an adventure of four level three players. Any suggestions? (besides players in that campaign)

~SE++

I'm a 3.x DM, but since that's quite close I'll give my system.

NPC Combat Block:

AC X; HP Y
Attack:
Full Attack:
Actions (Spells, action-needing special abilities, etc.):
Special Qualities:

Yeah, that's basically all you need for combat. I don't worry to much about the exacts of class and levels, which makes me pre-disposed to sorcerers, warlocks, warriors, etc. as the technical class of enemies. As such, my on-the-fly NPCs have "about" what you'd expect, but I don't do every step in creating a character.

That does cut my time down, so that works. Thanks.

But what about having four level three players? How many level "X" creatures should I throw their way at a single time for it to be difficult but have them survive? Can you try and have a single level 6 attack them, or will it overwhelm them? Would 8 level 1's be too much? These are the things I have little experience planning. What's an appropriate amount to toss their way and have a decent chance they'll survive?

~SE++

To echo what Tevish said, dont expend too much effort building and NPC with classes and levels as you would a player charcter. Usually you know the general powerband of where you want the NPC to sit, so take that and work for there.

I'll go more in depth when I find myself at my desktop.