For quite a while, OpenAI’s GPT-4 model was up to date until September 2021. However, recently, it appears that GPT-4 has been updated with an up-to-date dataset date of April 2023.
Knowing better than to trust what ChatGPT says, I tested this on ChatGPT web and different modes. I asked it with the default GPT-4 model and the Advanced Data Analysis model. I then checked GPT-3.5 as well.
GPT-4 on both web and mobile says April 2023. For GPT-3.5, it says January 2022.
Presumably, the updated dataset is in preparation for OpenAI’s dev day on November 6.