There were only three people marching, but often it seemed to the writer that there were four.
I think I have come to the most important paragraph in the book SOUTH by Ernest Shackleton. In my edition page 230.
With his two companions, Worseley and Crean, Earnest Shackleton is on the march across South Georgia.
He felt often that there was another person, but he kept quiet about it. Later on Worseley said that he felt the same. And Crean confessed to it too.
People who know their Bibles remember the story of the Fiery Furnace in the book of Daniel. Please read the verses 24 + 25 in Chapter three of Daniel.