Can’t Add / Edit Post WordPress

While WordPress post length is effectively unlimited I think any web request to update the text on such a huge page will be blocked by available memory and configured limits on the web server.

While it might be cumbersome, could you try updating the page content using phpMyAdmin or some other database tool? That should unblock you from these limitations.

“404 error” indicates failure to communicate with the server. There might be following reasons for this.
– Configuration issue
– Memory issue (contact your web host)
– Issue with Permalinks, check this:?How to fix 404 error when using Pretty Permalinks?
– Possible conflict with one of the plugins you are using: try?resetting the plugin folder?;?https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/
– Conflict with theme: try switching to the default theme for a moment and see if that resolves the issue.
– Check the server error log for your hosting account for any specific error message. Ask your web host for further assistance.