We have a 4 month old kitten that has recently started hissing at people when they come to the house. She has never done this before and has always been very friendly.
Does anyone have any idea why she might have started doing this and what we can do about it?
Cats are territorial animals, and as kittens mature, they develop a sense of territory, and they will defend it, usually by warning 'visitors' that they are there, and declaring the area as taken.
That's what your kitten is doing, and when she gets used to people coming and going, and realises they are no threat to her home, she'll settle down.
Just ignore her - she'll get past this reaction in her own time.