Epic Random Loot

Chests in Challenges are capped at CR 25. That's intentional. We didn't want Challenges to be the only viable way to get loot, which they would have been if you could get CR 30 chests in challenges, but only up to 25-26 from quests. To get the really special stuff, quests need to be run on Epic Elite, with loot boosts.

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There's a whole bunch of special prefixes and suffixes that have not been discovered yet, because they can currently only be found in endgame Epic Elite content while a loot boost is active.

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Secondly, are the +4 unbound tomes random loot or are they Elite only drops in very specific chests?

Both. They're in random loot in very high level tables, right where you'd expect to see them given the interval between +1, +2, and +3 tomes- they continue that same pattern.
Then they're also in Forgotten Realms non-raid quests on Epic Elite difficulty as bonus treasure that will occasionally appear.
+4 tomes are in the raid, Caught in the Web, on any difficulty, while Epic Elite also has [REDACTED], which I'm sure many of you have already guessed.