What happened with Chult and Ubtao during 4e? Ubtao's gone, and Dendar the Night Serpent was released and became a god. How and why did those events transpire?
The 4e FR Campaign Guide is completely silent about the topics you raise. Neither Dendar, nor Ubtao appear in its index. To the extent that I could track, Dendar is mentioned only one time in name as a primordial, and one more time as simply the Night Serpent while the Shadow Epoch (before -35000DR) is discussed. The Shadow Epoch is also the only time we hear about Ubtao the Deceiver, as the traitor who sided with the elder gods against fellow primordials. 4e FR Player's Guide has nothing at all regarding these topics; I could not identify a single instance where their names appear at all.
The main things we know from these two 4e books is that Chult became a separate island of its own after the Spellplague and the merging of Abeir and Toril. Mezro turned into a pit that was being haunted by various undead.
The 5e Sword Coast Adventurer's Guide is also thin in terms of information regarding Chult. Sadly the Second Sundering has not restored Mezro, it is still described as a ruined city haunted by the undead. Ubtao is not mentioned in the book (does not appear in the index), and Dendar is only mentioned as one of the possible warlock patrons; and is described to be from the Far Realm of the Great Old Ones.
In short, I don't believe there are RAW answers to your question(s) in the campaign settings. Dendar does not appear to have been released though, and it has not become a deity.
Finally, according to the recent 5e adventure Tomb of Annihilation, (spoilers ahead):
9 primal spirits came to be worshipped as gods after Ubtao's departure, but they were then destroyed by Greyhawk's infamous demilich Acererak, so it is unclear how much the FR designers want to continue the Ubtao-Dendar story for the future of Chult. However we also read that Ubtao left because of strife amongst his followers and their over-reliance on him, that yuanti are planning to open a portal to free the Night Serpent, and that Mezro planeshifted, so there may be more to come.
To add to ZwiQ's answer:
The DMs Guild Adept "Lost City of Mezro" line of supplements provide, not the full story, but tantalizing details about the Fate of Ubtao as well as the fates of Mezro and Artus Cimber. Mainly "Ruins of Mezro" and "Maze of Shadows" to be specific.
Then there are AL adventures that cover the release of Dendar the Night Serpent, and presumably how the end of the world is averted. I haven't read them myself but they are (in order):
Finally, the Power Guide's Tomb of Annihilation Companion suggests powers for the four legendary treasures described (but not statted up) by the hardcover adventure. Let's just say it would be bad news if one out of the four fell in Dendar's hands, as the old snake battles its way out of the underworld. :-)
PS. Do note that none of this is official first-party material, so I'm making no pretense of giving a RAW answer here. Consider this to be "helpful cruft" for those wanting more than what the previous answer can give. It's badly scattered, but if taken together, I would say it does hint at a "semi-offical" answer to the question asked, and related issues.