4 Players and Thunderspire Labyrinth

[keterys]

A difference of one player is roughly equivalent to a difference of one level. Either make sure the players are one level higher, or make sure the opposition is one level lower. I suspect you'll find that players would more happily be one level higher and that lowering everything by a level would be more annoying for the DM.

But whatever works for you.

Example:
4 level 5s facing
level n = 800xp
level n+1 = 1000xp
level n+2 = 1200xp
level n+3 = 1400xp

vs.
5 level 4s facing
level n = 875xp
level n+1 = 1000xp
level n+2 = 1250xp
level n+3 = 1500xp

Delta no more than 100xp (75,0,50,100). And it tends to work out pretty nicely for the PCs since they get missed more, hit more, so that part is satisfying.

 

[Llamas Notsheep]

It depends a lot on the players and their characters. I've run a group of 3 highly optimized characters through a number of the Scales of War adventures designed for their level (meaning skipping about every third adventure, since they advanced a lot more quickly). They rarely ran into trouble while going most of the way through heroic. I believe that group was a dwarf battlerager (before the latest update), a TWF ranger, and a taclord.

We also tried a couple of adventures with 4 optimized characters going through adventures designed for 2 levels higher than them. That group was a taclord, a battlerager, a crossbow ranger, and a reaper's touch cold sorc. They were in mid-late heroic and did fine. People dropped occasionally, and I provided a few extra healing potions in the treasure parcels, but they pulled through both adventures.

 

[Zinovia]

One thing that I would do with Thunderspire is to replace some of the normal creatures in there with 4-6 minions instead. Minions don't appear in any of the encounters in Thunderspire until near the end of that module, but it really would benefit from having some. You can bring them in either from the start, or as waves of reinforcements.