According to the sage advice an elf can get a long rest in 4 hours but must otherwise satisfy all requirements of a long rest. The base rules include 6 hours of sleep and no more than 2 hours of light activity which includes "standing watch".
All elven trance does is reduce the 6 hours of sleep to 4 hours of trance. They still need 4 hours of not standing watch.
All elven trance does is reduce the 6 hours of sleep to 4 hours of trance. They still need 4 hours of not standing watch.