select sentence, score from (
(
SELECT  MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) AS score,
	sentence_id, sentence
FROM articles.sentences 
WHERE MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE)
and MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) =4
order by rand()
limit 0,20
) union
(
SELECT  MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) AS score,
	sentence_id, sentence
FROM articles.sentences 
WHERE MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE)
and MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) =3
order by rand()
limit 0,20
)  union
(
SELECT  MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) AS score,
	sentence_id, sentence
FROM articles.sentences 
WHERE MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE)
and MATCH (sentence) AGAINST ('january february march april' IN BOOLEAN MODE) =2
order by rand()
limit 0,20
) 
) a
order by rand()


EXPLAIN:
1	PRIMARY	<derived2>	ALL	(null)	(null)	(null)	(null)	60	Using temporary; Using filesort
2	DERIVED	sentences	fulltext	fulltext_sentence	fulltext_sentence	0		1	Using where; Using temporary; Using filesort
3	UNCACHEABLE UNION	sentences	fulltext	fulltext_sentence	fulltext_sentence	0		1	Using where; Using temporary; Using filesort
4	UNCACHEABLE UNION	sentences	fulltext	fulltext_sentence	fulltext_sentence	0		1	Using where; Using temporary; Using filesort
(null)	UNION RESULT	<union2,3,4>	ALL	(null)	(null)	(null)	(null)	(null)