(未テスト!)のようなもの:
SELECT field1 FROM table1
WHERE field1 >= 4006
AND (field1 < (
SELECT field1
FROM table
WHERE field1 > 4006
AND field2 = false
ORDER BY field1
LIMIT 1
)
OR
NOT EXISTS (
SELECT field1
FROM table
WHERE field1 > 4006
AND field2 = false
ORDER BY field1
)
)