Hi there!

Thanks for your query.

SPPB 3 pages of Joomla 3 may not work properly in SPPB 4 with Joomla 4.

It will be better to upgrade the site from Joomla 3 to Joomla 4.Here is a blog post for your ease.

https://www.joomshaper.com/blog/how-to-update-your-site-to-joomla-4

Here are the export/import documentation for both SPPB 3 and 4.

SPPB 3: https://www.joomshaper.com/documentation/sp-page-builder/sp-page-builder-3/export-import-and-save-how-to-reuse-custom-designs

SPPB 4: https://www.joomshaper.com/documentation/sp-page-builder/sp-page-builder-4/export-import-and-save-for-later-use

Note: Please keep full site backup before trying anything, it's very much important.

Best Regards