A 4D being/thing staying in 3D was mentioned in the context of UFO crashes I think.
Windowfallers seem to be rather 3D, I don't recall reading something about a windowfaller having variable physicality; I am under the impression that they're simply beings from another world that involuntarily step through a "portal" and end up in our world.
So it seems they can be killed, although at least this specific beast was super resilient against human made ammunition.
On the day the family moved their furnishings onto the property, they had their first foreshadowing of the events that would follow. They spotted an extremely large wolf out in the pasture. The wolf cautiously made its way across the field, and, to the surprise of everyone, sidled up to the family, acting like it was a familiar pet. It had rained that day, and the family remembers the wolf smelled like a wet dog as they were petting it.
After a few minutes, the wolf strolled over to the corral and grabbed a calf by its snout, attempting to pull it through the corral bars. The rancher and his father began beating on the wolf's back with sticks but it wouldn't release the calf. The rancher grabbed a .357 Magnum from his truck and shot the wolf at point-blank range. The slug had no noticeable effect.
The rancher pumped another bullet into the wolf, which then let go of the calf but stood looking at the family as if nothing had happened. The rancher shot it two more times with the powerful handgun. The big animal backed off a bit, but showed no signs of distress, not even any blood.
The mystified rancher retrieved a hunting rifle and shot the wolf again, once more at close range. The rancher is not only an experienced marksman but a big-game hunter of considerable repute. Five slugs should have been enough to bring down an elk, let alone a wolf. The fifth shot caused a chunk of hair and flesh to fly off the wolf, but it still didn't seem fazed. After a sixth shot, the wolf casually trotted across the field into a muddy thicket. The rancher and his father tracked the beast for about a mile, following its pawprints through the mud, but the tracks suddenly ended, as if the wolf had simply vanished into thin air.
Ankh said:Apparently nothing would really happen were you to let a black-eyed thingus into your house; which is the case with most window fallers apparently - the reason it's so hard to have physical contact with them (e.g. can't shoot them, etc). It's talked about here:
Laura said:(Galatea) The black-eyed kids. I want to ask about them again. What do they do if you invite them into your house?
(L) What black-eyed kids?
(Galatea) The evil black-eyed children that knock on people's doors, and they ask to come in. They just scare people.
(Andromeda) What did we ask last time? What did they say?
(Galatea) They're window fallers. But what do they do if you actually let them in? Do they suck your energy or eat you?
(Perceval) They eat all your roasted chicken.
A: Not much!
Q: (L) If they're window fallers, they'd prolly come in and then disappear. Poof!
(Galatea) So they'd just walk in, and look all scary, and then that's it?
(L) Pretty much.
A: Yes.
(Galatea) Well, that's not fun!
(Perceval) That's what most of those window fallers do. They just stare at people and creep them out.
(L) And suck people's fear.
(Perceval) Every now and then, one of them might chew on a human or something, but mostly they just scare people.