Is it better do to a union in SQL or separate queries and then use php array_merge?

I have a SQL query that has 4 UNIONS and 4 LEFT JOINS. It is layed out as such:

SELECT ... FROM table1
    LEFT JOIN other_table1
UNION SELECT ... FROM table2
    LEFT JOIN other_table2
UNION SELECT ... other_table3
    LEFT JOIN other_table3
UNION SELECT ... FROM table4
    LEFT JOIN other_table4

Would it be better to run 4 separate queries and then merge the results with php after the fact? Or should I keep them together? Which would provide that fastest execution?