How to edit markup on existing pages using version 4 of the CKeditor library in Drupal 6? I installed/enabled CKeditor version 4 (from its download page) to use it in a Drupal 6 site. It's working fairly well with one exception: editing markup on existing pages doesn't work.

What happens is, that old pages won't accept the changes. It's baffling: the code is correct and it shows up in the source, but it's not rendering on the page for nodes that were created before I installed CKeditor 4. For example, I have a line with a line break at the end of it, but the line breaks aren't showing up.

But for new pages, the line break works just fine. It's only the old nodes where it's not showing up.

And if I uninstalled CKeditor version 4 and go back to CKeditor version 3, the issue disappears.

Any ideas or recommendations?
CKEditor Version 4.x is known to have 'some' issues, at least when used in DRupal. The issue you described in the question here is probably one of them. And it doesn't matter if you use the CKEditor 'module', or the WYSIWYG 'module' (in combination with the CKEditor 'JS library'). Because for both modules these issues exist.

In my experience, going back to CKEditor 3.6.x version (or never upgrading from 3.x to 4.x) has always been the best "compromise", since in the 3.x versions these issues seem to not exist.