If you are migrating from an earlier version than 4.0, and your database contains statistics collected using a version earlier than 2.2, you will probably encounter an issue during our automated database migration. To avoid that issue, you will need to clean up your statistics by executing some SQL commands described in issue XWIKI-8129. Be sure to apply these before attempting the migration. If your are in doubt, apply it, these could not hurt. If you don't, you will face duplicate IDs errors during the migration process, and it will fail, preventing you to run your wiki.

If you are upgrading to this version from an older version which uses Lucene 3.x, you have to manually delete the lucene directory in the xwiki data directory. For example, that could cause Lucene to loop endlessly while trying to index pages, among other things. This is caused by  the Lucene migration to 4.0 (see XWIKI-8404).

PLEASE, BACKUP YOUR DATABASE BEFORE STARTING YOUR WIKI.
AUTOMATED MIGRATION PROCEDURE TAKE PLACE AT STARTUP AND YOU MAY NEED TO START OVER IF YOU ENCOUNTER AN UNEXPECTED ISSUE.

Always make sure you compare your xwiki.cfg and xwiki.properties files with the newest version since some configuration parameters were added. Note that you should add xwiki.store.migration=1 so that XWiki will attempt to automatically migrate your current database to the new schema. Make sure you backup your Database before doing anything.

If you are migrating from an earlier version than 4.0, and your database contains statistics collected using a version earlier than 2.2, you will probably encounter an issue during our automated database migration. To avoid that issue, you will need to clean up your statistics by executing some SQL commands described in issue XWIKI-8129. Be sure to apply these before attempting the migration. If your are in doubt, apply it, these could not hurt. If you don't, you will face duplicate IDs errors during the migration process, and it will fail, preventing you to run your wiki.

If you have encountered an issue with the migration, you may need to start over from a backup. Be careful that during the migration, two new tables have been created (in each xwiki database for a farm) and should be dropped when you restore a backup of a database coming from a version earlier than 4.x. These two tables are DATABASECHANGELOG and DATABASECHANGELOGLOCK. These should only be removed if you restore a pre-4.x database schema. Mixing an old database with these two tables will surely cause issue during schema updates (liquibase checksum failure).