Let's say I'm writing a document with 6 chapters. I want to force the table of contents to be split between two pages. The first page would list the first 4 chapters, the second page would list the last 2 chapters.

How can I do that?

If it matters, assume the document class is report.

Within the body text, right before the fifth chapter, you could add a line break to the .toc file, for example:
\addtocontents{toc}{\protect\newpage}