“Sorry! Forgot you guys have to obey the Fourth Wall and everything. Pretend you didn't hear that.”
—Bill Cipher, Destroy the Godmodder 2
Fourth Walls are powerful artifacts that were introduced in Destroy the Godmodder 2. They take the form of giant versions of fenestrated walls from Homestuck. There is one centered near every universe in Fiction, and they lie in the Void. The Fourth Walls separate the universe from the outside of the Void, and other universes, acting as a barrier and a continuity establisher.
The Fourth Wall of Minecraft was used in Destroy the Godmodder 2 by the Homestuck Invasion, who broke it and Homestuck's Fourth Wall in order to enter GodCraft. Minecraft's Fourth Wall was later repaired and broken again near the End of Act 4 by the Employer.
Fourth Walls are able to show events happening in any universe on their surfaces. This may be tied into their status as keepers of universes. As stated above, they act as barriers between a universe and the rest of the Void, and possess large quantities of continuity power.
Although Fourth Walls are powerful, they have their limits. They can be broken with enough force. Depending on the force used, a hole can be made through the Wall, or it could be shattered completely. Depending on the severity of the break, the universe the Wall belongs to may start to be influenced by the Void and other universes, which could, at best, lead to crossovers, and, at worst, lead to different universes invading.
Such an event would be very hard to fix. A known solution is to initiate a temporal anomaly that is used in conjunction with the energies of creation, also known as a Scratch.
Minecraft's Fourth Wall was first broken by the Godmodder in between DTG and DTG2, when he was flung into the Void by Crusher48. This resulted in Homestuck's Fourth Wall getting broken as well, creating a small passage between the two. The Fourth Wall would be revisited later by the Homestuck Invasion, who would break both walls to enter GodCraft.
The Fourth Wall stayed broken for the entirety of Act 2 until GodCraft's First Block was Scratched, creating a temporal anomaly that restored the Fourth Wall to its original state, banishing all of the Homestuck Invasion from GodCraft. The Fourth Wall, now continually powered by the First Block, remained indestructible for nearly two whole Acts until Doc Scratch took the Disc of Mojang that powered it, leaving the First Block vulnerable.