hi,

As i can see is the second index use will reduce the cost drastically. As far as rules concerned, for a low cardinal field and if mostly u r using or predicate frequently then u should go for the Bitmap index. As u said these four values r mostly used then what i will suggest is to use a bitmap index.

More over , if see the execution plan the range scan value that is using the second index is more prefered. What u can do is , drop the b-tree(normal) index then run the query to find out the resource cost.

All the best.