Nobody has so far commented on the four distinct values in your column. In one of Gail's articles that you're about to read, it says this:
It usually works out that the number of rows where the optimiser decides that key/RID lookups are too expensive is somewhere around 0.5%-1% of the total rows in the table
Therefore your index isn't likely to be very useful, unless the four values are massively unevenly distributed, or unless the index can be used as a covering index.
John