Stanza 4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Where dwell the Ghouls,—

But wait, things get worse! Apparently this nasty swamp is the home of the Ghouls! We don't know who they are yet, but it sure doesn't sound good.
Originally, a ghoul meant an evil creature that eats dead bodies, but it can also mean any sort of fiendish, horrible spirit. Definitely not the critters you'd want to run into in a swamp.
Poe makes their entrance really dramatic, too. The Ghouls show up almost exactly in the middle of the poem, and the line is really short (almost half the normal length). Do you feel how you sort of trip over it when you read through the poem? This line is really meant to jump out at you.

By each spot the most unholy—
In each nook most melancholy—

Apparently these Ghouls are everywhere, in every nook and cranny.
Be sure to keep an eye on the words that Poe is using here, like "unholy" and "melancholy." Every line is made to fill you with a feeling of evil and sadness and despair. That's Poe's trademark, and he really goes for broke in these lines.

There the traveller meets aghast
Sheeted Memories of the Past—

The land of the Ghouls is a place of death and despair, and also of ghosts.
In this dark and scary swamp, travelers (like us!) are aghast (terrified and filled with horror) to meet with "Sheeted Memories of The Past." What exactly are they? Well, we'll find out more soon, but already we can tell that they must be pretty scary, since he's set us up with all that grungy swamp stuff.
The word "sheeted" also makes us think of spirits and ghosts, right? You probably spent at least one Halloween under a white bed sheet pretending to be a ghost.

Shrouded forms that start and sigh
As they pass the wanderer by—

Now we learn a little more about the "sheeted memories" we met in line 34. They are "shrouded," which means the same thing as sheeted. Basically just wrapped in cloth, only scarier, because a shroud is a cloth you wrap around a dead body.
They "start" (jump with surprise) and sigh, when they walk by you.
You can picture it now, right? A swamp filled with restless, moaning ghosts. Spookier and spookier….

White—robed forms of friends long given,
In agony, to the Earth— and Heaven.

Poe starts this line by telling us, for the third time, though in a slightly different way, that these ghosts are draped in fabric ("white-robed"). We're not quite sure why that's such a big deal.
Right after that, though, he tells us something super-important. The speaker recognizes these ghosts! They appear in the shape of dead friends, people who have been buried ("given/ […] to the Earth") and gone to Heaven.
We think line 38 could be the emotional core of the poem, the moment when we really understand why this guy is so sad, and why this trip to Dream-Land actually matters.