As the leader of the witch side, Battler will be forced to continue the serial murders. So he must carry out 13 murders using the game pieces. If he can create a crime that follows the rules of Beatrice's game board and is seen by Lambadelta and Bernkastel, they'll be able to confirm that he understands this world, and the game will be over.
Ryuukishi, jokingly, said that he might even end with EP6...but he also said that ending on either EP7 or EP9 would even be possible as well. In other words, "no comment"
What is Solvable:
It is possible to solve the "who dunnit", "why dunnit", and "how dunnit" for up to EP4 at least, but this is from Ryuukishi's perspective. Just because it would be possible for him doesn't mean that it will be possible for all the readers, coming from their own perspectives.
Some people have already solved about 98% of the epitaph. For the rest, you'd need to be on the actual island. However, if the people who have solved 98% went to Rokkenjima, they would be able to solve the rest in about an hour.
Several people have already guessed the true location of the gold.
In both EP6 and EP7, the full process of solving the epitaph's riddle will be revealed. You'll hear about it bit by bit from characters in the game.
In EP5, the game stopped shortly after Hideyoshi's corspe was discovered, but people who know the "answer" should be able to predict what happens next in the second twilight and beyond.
If you know the "answer", you should be able to understand what gold text is, what conditions it can be used in, and whether it's stronger than red text or not.
If someone knows the answer of the game, they can use the gold even if they aren't the Game Master.
Erika does not know the truth, but Lambdadelta can probably still use the gold.
Beato managed to communicate everything she wanted to Battler at the end of EP4.
Episode 6 will probably focus more on 'story' than 'mystery'