Final Fantasy 15 has already gotten more after release support than most games get, but Square Enix is keeping the good times rolling with four more DLC episodes. According to FFXV director Hajime Tabata, in addition to the three previously announced episodes that will be coming in 2018, plans for a fourth have been added.
Tabata said that when thinking with the Final Fantasy 15 team on what new things they wanted to finish out the game with, it became apparent that they needed to add a fourth new episode. We know the release of these four episodes will likely extend until the end of 2018, but there's a chance that some of the DLC might not be ready until next year.
The team behind FFXV doesn't want to rush the new episodes and intends to prepare them with care. No firm release date has been announced for any of the four new episodes so there's a possibility we might not see the last one until we're into 2019.
We know that the first new episode to release for Final Fantasy 15 will focus on Ardyn, but Tabata and Square Enix haven't let the cat out of the bag on who the other three will be. When asked by Dengeki Online whether or not there will be an Episode: Lunafreya, Tabata said, "Well, she does appear of the box-art of the Royal Edition."
Since Lunafreya was basically AFK the whole game, it would be nice to see the events of Final Fantasy 15 from her perspective. In addition to her and Ardyn, I imagine the likely candidates for the other two episodes are Aranea and Iris. Those four episodes would provide some backstory and closure for some fan-favorite characters and seem like they'd be an excellent way to close out the game's story with.
As stated above, we have no idea when any of the four upcoming DLC episodes will release. However, it seems like it will at least be after the release of Final Fantasy 15 Royal Edition and Final Fantasy 15 Windows Edition on March 6.