The 4000 character limit of Case Comments is set by Salesforce and cannot be directly increased.

That said, if your organization would like to send comments longer than 4000 characters please refer to the Email to Case Premium Enhanced Comment Editor.

When the Enhanced Comment Editor is enabled for the New Comment page, Email to Case Premium populates a field called Most Recent Public Comment (HTML) upon Save/Send. This field is able to hold up to 32,768 characters.

Note:

• The 32,768 character limit includes email recipients.
• Formatted text / images will consume a larger amount of character space than plain text.

In the event a comment is over 4000 characters, the comment will continue to be truncated when viewed from the Case Comments related list (or Case Presentation Layer). However, if the template being used for the customer notification includes the Most Recent Public Comment (HTML) field, the customer will receive the full message entered on the New Comment page up to 32,768 characters.