**Recent Errata has sheds further light on the issue.**

The [PHB errata Update from 08/31/2017][1]

> **Long Rest (p. 186)**. The first sentence of the rule now reads, “A long rest is a period of extended downtime, at least 8 hours long,
> during which a character sleeps for at least 6 hours and performs no
> more than 2 hours of light activity, such as reading, talking, eating,
> or standing watch.” In addition, you regain at least one Hit Die when
> you finish a long rest.

Additionally, while not Errata, the [Sage Advice Compendium][2] has updated its guidance on the Trance trait to account for the new Long Rest rules wording. 

> **Does the Trance trait allow an elf to finish a long rest in 4 hours?**
> If an elf meditates during a long rest (as described in the Trance
> trait), the elf finishes the rest after only 4 hours. A meditating elf
> otherwise follows all the rules for a long rest; only the duration is
> changed. [This answer has been altered as a result of a tweak to the
> rules for a long rest, which appears in newer printings of the
> Player’s Handbook.]

**Elves Can complete a long rest in 4 hours, warforged need 8 hours, but less *sleep*** 

If you follow this advice from the game designer than I would have Elves be able to complete a long rest in 4 hours if they spend it in a trance with all the other normal rules for long rest applying (except sleeping 6 hours).

Warforged still need to rest for the full 8 hour period, but they can sleep for only 4 hours in their sleep-mode vs 6 hours races other than elves require. 

  [1]: http://media.wizards.com/2017/dnd/downloads/PH-Errata.pdf
  [2]: http://media.wizards.com/2017/dnd/downloads/SA-Compendium.pdf