1) Any events created in EE4 will not be available in EE3. These would need to be manually created again along with any registrations.
2) Some of the critical shortcodes for the four WordPress pages are different and they would need to be updated. When EE3 is running, you can find these in Event Espresso –> General Settings –> Page Settings.
Again, try my idea here first as this is a display issue related to archive pages:
The support post ‘EE4 back to EE3’ is closed to new replies.
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Support forum for Event Espresso 3 and Event Espresso 4.