The three states below have something in common, and there is a fourth state that can be added to the list. Can you determine what the states have in common and what the fourth state is?

OKLAHOMA
HAWAII
INDIANA

HINT:
The answer involves each state's capital city.

Answer:

The three states above each have a capital city whose name begins with the same letter as the state's:

OKLAHOMA - OKLAHOMA CITY
HAWAII - HONOLULU
INDIANA - INDIANAPOLIS

The fourth state is DELAWARE - capital city DOVER.