Fulltext search using multiple indexes
Unfortunately, this is how MySQL Query Optimizer treats FULLTEXT indexes. When a MATCH clause is the only clause in the WHERE, the index will be used. When used in conjunction with AND, the index may easily get overlooked.

I wrote about this behavior before in Mysql fulltext search my.cnf optimization

SUGGESTION : Rewrite the query as the union of two FULLTEXT searches

SELECT * FROM content 
WHERE MATCH(a, b, c) AGAINST ('keyword')
UNION
SELECT * FROM content 
WHERE MATCH(d) AGAINST ('keyword');


GIVE IT A TRY !!!