**Both races receive the benefits of a Long Rest in only 4 hours**

In the rules quote about the Elf Feature **Trance** it explicitly states that you gain the same benefits as a human does from 8 hours of sleep. Since Long Rest specifies a minimum of 8 hours the description of **Trance** in the rules would then mean that by meditating for 4 hours an Elf receives the exact benefits of resting for 8 hours aka a long rest. 

While the warforged **Living Construct** feature is less explicit, its wording is very close to that of the Elf **Trance** feature and just on the D&D Next rules alone should work the same way.

**In 4e Warforged  have an identical feature**

In 4e what D&D Next calls a Long Rest is called an Extended Rest.

Warforged have the Unsleeping Watcher feature.

>**Unsleeping Watcher:** You do not sleep and instead enter a state of inactivity for 4 hours to gain the benefits of an extended rest. While in this state, you are fully aware of your surroundings and notice approaching enemies and other events as normal.

**In 4e only Drow have the Trance Feature**

>Trance (Drow): Rather than sleep, drow enter a meditative state known as trance. You need to spend 4 hours in this state to gain the same benefits other races gain from taking a 6-hour extended rest. While in a trance, you are fully aware of your surroundings and notice approaching enemies and other events as normal.