SELECT
Posts.ID
FROM
Posts
INNER JOIN Votes ON Posts.ID = Votes.PostID
WHERE
Posts.PostTypeID = 1
AND Posts.Tags LIKE '%C#%'
AND Votes.VoteTypeID = 6
GROUP BY
Posts.ID
HAVING
COUNT(DISTINCT Votes.ID) > 4
This takes a while to run so could probably be optimized. The Tag filter could probably use full text search which may be faster if it is enabled.