No. Imaginary friends are creations of the mind, usually by children. They are not the same as ghosts in ghost stories. Ghosts in stories are often depicted as the spirits of the dead, while imaginary friends are just figments of a person's imagination and don't have the same spooky connotations related to death and the afterlife.
In a small town, a boy named Mark had a friend named Alex. Alex died in an accident. Mark was so sad. One night, Mark woke up to find Alex's ghost sitting on his bed. They began to talk as if nothing had happened. Alex's ghost told Mark that he was still around, just in a different form. They continued their friendship. Mark would go to the places they used to go together, and he could feel Alex's presence. Their ghostly friendship became a special bond that Mark cherished.
Well, Thomas and Friends Ghost Stories might be quite interesting. It could involve the trains in the Thomas and Friends world having some spooky adventures. Maybe there are tales of mysterious happenings around the tracks at night.
The theme of acceptance is quite common. The living character has to accept the existence of their ghost friend and the fact that the world is more complex than they thought. Also, there is often a theme of learning. The living friend might learn about the afterlife, or different perspectives on life from the ghost friend. For instance, a ghost friend might show the living friend how precious life is by sharing their regrets from when they were alive.
The four friends might be motivated by a desire to stand out and be different. Writing ghost stories gives them a chance to show their imagination and entertain others. They could also be influenced by a recent spooky event or a book or movie that sparked their interest in the paranormal.
Imaginary friends are usually friendly and created for companionship. Characters in ghost stories are often spooky. Imaginary friends are more personal, like a secret friend only one person can see. Ghost story characters are meant to be shared in a scary story.