How to unlock / Reach
First you need to ''win'' (defeat the Act 3 Boss) with all characters at least once. Then, upon starting a new run with any character, you will notice a strange triangular symbol next to your name—this is where the keys will go. There are 3 keys: the Sapphire Key, the Ruby Key and the Emerald Key.
To get the Ruby Key, you need to go to a rest place and there will be an option called ''Recall'' to acquire it.
To get the Emerald Key, you must search the map for a ''marked'' Elite fight, then defeat it. You will receive the normal payout, plus the key.
To get the Sapphire Key, you need to find a chest. The key is linked with the item, meaning you can only take one (essentially, you are forgoing a relic). Now, once you complete Act 3, you will go to Act 4. This can be done in any Ascension.
The three keys need to be captured on each new run in order to access act 4. However, you don't have to take them at the first opportunity. For example, the key you get instead of resting/upgrading could be obtained at any campfire. There is one elite in every act that will be tagged as the "special elite". ANY non-boss chest (including ? chests) can get the third key (so you can wait for a useless relic one).
The Act 4 map has one path, a fire, a shop, an elite fight and then the final boss.
The Elite fight is against the Spire Shield and the Spire Spear.
The Boss fight is against the Corrupt Heart.