the feature states 'The sphere vanishes at the end of the rest or when you leave the sphere.'
Does an elf end their rest after their trance - 'Elves don’t need to sleep. Instead, they meditate deeply, remaining semiconscious, for 4 hours a day. After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.' or does the rest still count for all 8 hours? ' A Long Rest is a period of extended downtime, at least 8 hours long'

I don’t think the intent is that trance (or any rest) is limited to any maximum amount of time. The long rest rules even say “at least”. So an elf that goes into a trance can still be long resting after those 4 hours are up. 

With that being said, the new rules for trance in the MMotM use “can” making it even more clear that the length of the trance is a choice. 

But honestly, from the way the feature is written, it is likely meant to apply to the entire group’s long rest anyway, no matter how long any single member is resting. 

Thanks for reply and I think that makes sense. I agree, it wouldn't really make sense for the feature to be limited like that, just for those playing as an elf