Hi there!
Sorry to hear that.
Actually, the SPPB 4 version is not supported anymore. It will be better if you update your SPPB version from 4 to 5 first then export/import.
You can reuse exported pages also on different websites, but you have to be sure that it contains the same list of addons and image structure. If not, part of the content may not work. Exported .json file does not contain any addon code inside, only layout and page settings. Also, you need to have the same images stored in the new destination to get the visuals.
https://www.joomshaper.com/documentation/sp-page-builder/export-import-and-save-for-later-use
Best Regards