reversereality said:Damn, to think that nobody provided an answer.. XD
Well as for episode 4, you need to know that the time loop event that the kids get in happens in the Daze. The monster (Azami) has a human husband, so he is destined to die at a relatively 'young' age. Same goes for her daughter Shion, who really isn't immortal. So Azami creates this world called the Daze or Heat-Haze where there is no flow of time, and plans to take her family to the Daze so that they may live forever and ever. In episode 4, when Hibiya wakes up, notice that his clock has no hands. This means that there is no measured time and strongly implies that he wakes up in the Daze.
Now Azami's daughter and granddaughter (Shion and Mary) died in August 15th many many years ago. And because of that, Azami commanded the Daze to swallow them up. Since the evil Snake made the Daze a living hell, Azami had no choice but to save her family from the suffering. So she gives Mary the Queen Snake AKA Combining Eyes Snake so that Mary can continue to live.
Since Azami gave up her Queen Snake to Mary, she lost control of the Daze and the command that she gave to it-- bringing in those who died August 15-- cannot be changed.
In episode 5, we see Hiyori, Hibiya, Konoha, and Shintaro get involved in a street accident. And when they die, they get sucked into the Daze. In order for one to escape the Daze, one has to obtain a snake from Azami. The snake basically keeps the person alive (that is why Mary is alive because she has the Queen Snake). Since Konoha already has the Awakening Eyes, and Shintaro has the Retaining Eyes, they can escape.
But Hibiya and Hiyori don't have a snake, so they can't escape.
But why can't both obtain snakes and get back? Well, there is only one snake left to give. Here's the list of snakes that inhabited their hosts prior to episode 4.
Note: The eye ability Staring Eyes is only inherited by the medusa bloodline and is an inherent ability. I believe that it is NOT a snake that grants medusas this power.
1) Combining Eyes Snake - Mary
2) Concealing Eyes Snake - Kido
3) Eye Stealing Snake - Seto
4) Deceiving Eyes Snake - Kano
5) Captivating Eyes Snake - Momo
6) Opening Eyes Snake - Ene
7) Retaining Eyes Snake - Shintaro
8) Eye Focusing Snake - ???
9) Awakening Eyes Snake - Konoha
10) Clearing Eyes Snake (the evil Snake) - Kenjirou
11) Favouring Eyes Snake - Ayano
Since there is only one snake left out of the 11, then only one person can leave the Daze. That is why one kid can leave, but not the other. So in order for one of them to get back, the other has to basically 'die' in order to decide who escapes the Daze with the Eye Focusing Snake. Thing is, the kids don't know this, and when one of them dies, the other does the test again and again and again in an attempt to save the other. In the end, Hibiya is saved due to Hiyori's sacrifice.