Ancient Aliens Season 1 Episode 4 (Ancient Aliens s01e04) “Closer Encounters” airs on Tuesday, May 18th 2010 at 08:00 pm via History.
Here is summary for Ancient Aliens Season 1 Episode 4 :

UFO sightings depicted in visual and written accounts are examined. Included: a 13th-century book describing a flying craft over Bristol, England. Meanwhile, a log entry from Christopher Columbus’ voyage to America recalling strange lights in the sky.

Few hours after original series aired on TV, online streaming video for Ancient Aliens Season 1 Episode 4 usually available on Net and of course, if video available, as soon as possible will show you where you can watch this episode for free.