low cardinality and highly concurrent db

We have one column with low cardinality, 4 or 5 unique values across 50 mil rows.

Our query has this colunmn as a predicate. Binary index is not helping. I am tempted to create bitmap index but the general myth is there could be lot of contentions. We have a highly active OLTP system with concurrent DMLs.

When people say that contentions due to bitmap indexes are massive, what excatly does it mean? I am not convinced that it locks the entire table.

Can anyone share their experiences with bitmap indexes in OLTP systems ?. I want to know % degradation during DML due to bitmap index if possible.

Thanks a lot for any advice