Need help on creating index for a query w/ 4 greater-than conditions

What sort of indexing I should use if I try to run the following query?

SELECT part_id
FROM part
WHERE dimen1 >= ? and dimen2 >= ? and dimen3 >= ? and dimen4 >= ?
ORDER BY dimen1, dimen2, dimen3, dimen4
LIMIT 100;

I tried creating index on the dimen1, dimen2, dimen3, and dimen4 columns,
but this dosn't help at all. The query plan I always get is sorting
followed by a seq scan.

Sort (cost=2822.86 rows=622 width=96)
-> Seq Scan on part (cost=2822.86 rows=622 width=96)

Thanks