D.B.
This is the age for "scary night monsters/shadows' as preschoolers' brains are developing the capacity for imagination. Reading books together before bed can help. The books help the child see that the fear can be worked through as the kid in the book has done. Here are some books kids 3-8 respond to:
Can't You Sleep, Little Bear? by Martin Waddell
Night Light; A story for Children Afraid of the Dark by Jack Dutro
Scary Nights by Irene & Paul Marcus
There's a Nightmare in My Closet by Mercer Mayer
Who's Afraid of the Dark? by Crosby Bonsall
Furlie Cat by Berniece Freshcet.
Local libraries and bookstores will have them. Good luck.