The main difference between ChatGPT 3.5 and ChatGPT 4 is the size and complexity of the models. ChatGPT 4 is a more advanced version of the language model and has been trained on a larger dataset, making it more accurate and capable of generating more complex and sophisticated responses. ChatGPT 4 also incorporates advancements in natural language processing (NLP) technology, such as better word embeddings, improved model architecture, and more effective training techniques. This enables it to generate more coherent and contextually relevant responses to a wider range of inputs.