DISTINCT not using an index
If I saw a data model where a table had 2.1 million rows, one of the columns had only 5 distinct values, and I knew that people wanted to get a listing of those distinct values on a somewhat regular basis, I would strongly suspect that the data model was missing a lookup/ dimension table.  Rather than trying to tune your current query, I would bet that the better answer would be to create a separate table that has just those 5 distinct values, create a foreign key relationship between your current table and the new lookup table, and then modify your query to hit the new lookup table instead.