The typical behavior of most browsers, to my knowledge, is to simply truncate the oldest data that does not fit.

For example, create cookies 1 through 9. When creating cookie 10 and the data size is going to overflow, cookie 1 is simply discarded.

In general practice, if you are worried about bumping into the limit and loosing cookies to overflow, it is probably time to rethink your strategy of what you are storing and start caching the data server-side and limiting the cookie to a value to access the cached data.