Use strong imagery. For example, 'Blood on white sheet'. Pick words that evoke fear like 'dark', 'death', 'lost'.
To create a good '4 word horror story', start with a spooky concept. It could be a haunted place like 'Cellar door creaks'. Then, use words that are associated with fear, such as 'alone' or 'cold'. Also, play with the unexpected. Instead of 'Sunshine and flowers', you can have 'Sunshine then shadow'. This contrast can create a sense of unease in just four words. Another tip is to use words related to the senses. 'Smell of decay' can be very effective as it makes the reader imagine a foul smell which is often associated with something dead or rotting.
First, think about common horror elements like monsters, the supernatural, or dangerous situations. Then, condense it into four words. For instance, 'Monster under bed'. You can also use ambiguity to your advantage. 'Whisper not mine' leaves the reader wondering who is whispering and why it's not the speaker's. And don't be afraid to use simple but powerful words. 'Dead eyes stare' is a great example. It immediately creates a vivid and terrifying image in the reader's mind.