There was talk that SB might go beyond to 4.6 or 4.7 way back at launch...
It all depends on how they want to space out the rest of the content.

Right now we can assume there will at least be:

a 3rd and final installment of the Return to Ivalice story a 3rd 24-man instance.

The Four Lords storyline has one more lord, Seiryu, we will most likely fight, plus the possibility that we might face Kohryu if things go awry. If they go the latter route then Seiryu would most likely be a dungeon boss (like Genbu) and Kohryu would be the final trial in that series.

United SB Beast Tribe story/quests

MSQ story wrap-up and tie-in to 5.0


Other things still on the table:
I've read speculation that Pyros might not be the final part of Eureka... we'll know more after 4.55 drops. In the previous expansion HW, the relics final step was equal in ilvl to the 8-man raid weapons. So if the Relic doesn't get to i405 in Pyros, it's possible that a final area will be implemented for the final step...
It's doubtful to me that we'll get a "victory lap" of content outside of Eureka for the 4.0 Relic, but I guess they could do a Victory Lap that requires you to go through all the Eureka areas and kill specific FATE targets with your 4.0 Relic equipped... /pray for those that have to defeat Cassie... If such a Victory Lap is a step in the 4.0 relic, then a 4th part of Eureka would be unlikely.

A 3rd Ultimate fight is a possibility as well...


The Seiryu/Kohryu part of the Four Lords kind of mucks it all up. So far in we've gotten only ONE new dungeon on odd patches and TWO new dungeons on even patches. If Seiryu is a dungeon boss, then that would need to be the ONE dungeon for 4.5... and then Kohryu would be the trial. Then Return to Ivalice and United Beast Tribe get rolled in for extra content...
This doesn't leave space for MSQ wrap-up and tie-in to 5.0... since that will most likely be a dungeon as well. It also doesn't leave room for a 3rd Ultimate to be slotted in, if that is actually in the works.

Of course, Seiryu could just be the final trial and Kohryu doesn't get fought, which leaves the 4.5 dungeon free for MSQ.