I didn't know about the 4000 tokens limit. Seems like it's better to have different chat sessions.
That limit is only a concern if you want it to analyze a document you provide. Once you exceed that limit, it is no longer analyzing the original document. It is analyzing its own summary of the original document, so information may be missing or errant in the analysis.
There is also a 2,500 +/- token limit per prompt. So even if you have a 4,000-word document to analyze, you have to give it in two pieces and instruct ChatGPT to concatenate them into a single document for analysis.