After resting up and defeating a lone Drow sent for some sort of reconnaissance, the party tried one more time to delve through the dungeon. Unfortunately, the inhabitants were simply too powerful. The party did manage to defeat them, but nearly at the cost of Eldon's and Nova's lives. After retreating once more, the party meditated on their skills, what they've learned, and feel ready to give this dungeon one last go.
DM Note to Players
There's a certain way to calculate the difficulty of an encounter. I have been scaling encounters for 6x 4th level characters, thinking that missing a player or two wouldn't make much of a difference. Well… without going too deep into the math, the "adjusted XP" should not have much exceeded 2,000 for one encounter; this last session clocked in at 5,400 and the next would have been 4,425! (No wonder you were having difficulty!)
So that's on me for making assumptions – I'm very sorry. However, as it turns out, advancing to level 5 solves this problem very neatly. I intended to wait until after this adventure to level up, but this is really the simplest solution.
So, if you haven't already done so, advance your character to level 5!
And finally, I broke down and bought dynamic lighting ($5/mo). It should be pretty awesome, I'm really excited to play with it! If you have some free time, I encourage you to go to Roll20 and play around with the dynamic lighting (I have set up a map for everyone to play with).