SELECt m.field,
COALESCE(s1.field, s2.field, s3.field, s4.field) AS field,
…
FROM main_table m
LEFT JOIN slave1_table s1 ON …
LEFT JOIN slave2_table s2 ON …
LEFT JOIN slave3_table s3 ON …
LEFT JOIN slave4_table s4 ON …

SELECt m.field,
COALESCE(s1.field, s2.field, s3.field, s4.field) AS field,
…
FROM main_table m
LEFT JOIN slave1_table s1 ON …
LEFT JOIN slave2_table s2 ON …
LEFT JOIN slave3_table s3 ON …
LEFT JOIN slave4_table s4 ON …