Get top 10 records from two identical tables
Assuming you don't care which result you get in ties, you can use ORDER BY / LIMIT twice, once in each subquery and then a third time in the main query.

This will be more efficient than a normal UNION query from two tables and then LIMIt, as it can use indexes on (Date) on both tables:

    ( SELECT ID, UserID, Date 
      FROM table1 
      ORDER BY Date 
      LIMIT 4
    ) 
UNION ALL
    ( SELECT ID, UserID, Date 
      FROM table2 
      ORDER BY Date 
      LIMIT 4
    ) 
ORDER BY Date 
LIMIT 4 ;