While I generally would not recommend keeping a four-year-old on a bottle, depending on the nature of your niece's autism this may not be unreasonable. Some children who have autism have feeding difficulties in addition to language and social interaction abnormalities. They may not eat appropriate foods or swallow certain textures without difficulty. Usually these feeding difficulties can be addressed with appropriate specific behavioral training, but until the child is able to do it on her own she needs to get adequate nutrition in whatever way works. For some children this may mean a bottle containing special formula or else foods that have been through the blender. With therapy, as the child learns how to eat regular food, the amount given by bottle can be limited gradually and eventually eliminated completely.
It is also possible that there is no specific feeding issue, and the girl is receiving the bottle out of habit. For some autistic children this may impair further their ability to interact with their surrounding environment, but for others it may soothe them enough to allow them to interact in a more meaningful way. There is no way for me (or you) to know what the circumstances of her bottle use are without speaking directly with her parents. You certainly could ask your sister in a non-judgmental way, why it is that her daughter still requires a bottle. You must then be willing to listen to her concerns and opinions, and help support her in whatever way may be useful. Parenting an autistic child can be overwhelming, and most families need some help. You can get more information about autism and related disorders (known as pervasive developmental disorder or PDD) from reputable websites such as the Autism Society of America or the National Institute of Mental Health.