I will put this as calmly, clearly, and directly as I can.
DON'T GO!!!
I've seen the horror that is Nightfang Spire. And I'm quite serious in saying what I said. Here's why:
1. Your party is small, and lacks the clerical punch to have even a prayer of surviving let alone winning.
2. The module, to be blunt, stinks. (Edited for Eric's Grandma.) I kid you not - it's a poor module, that is overpowered for the levels it's supposed to be for. Worse yet, it's just plain mean. It's arbitrary and cruel and death will come swiftly and often. My group had 6 PC's including a cleric undead-hater, and we still got stomped. If the DM plays the monsters with any remote sense of intelligence, they will destroy you.
Now of course, all this changes if the DM adapts the module. But the real crime here is that DM's are deceived by it. They pick it up, and read through it, and because it's WoTC published, think that it should work fine for their group. It's not til the players are inside, suffering senseless save-or-die traps and repeated attacks that they realize that it's a badly scaled horror of a place. The final encounter quite frankly seems unwinable to me.
If you really truly must go there, then please please please please talk to your DM. There are reviews and story hours that used the module on the boards. Tell him people have said it's a bad module and not only overpowered but also deceptive. Beg him to take a long serious look at the module and the encounters and judge if it's a good fit.
Good luck. You'll need it.