is database4 the only database which have the problem OR there are other database as well.

Just try to rename the mdf and ldf file of the database. Are you able to rename it with any other name or getting the error as file is in use.

If getting error in rename then it still being accessed by sql server.

Just run the query in master :-

select * From sysdatabases and check the status.

And recovery option(when all hope gone), try restoring with latest backup set with different name. But before this I will really like to see the errors you getting in above step to resovle the case further