Number 4 is incorrect, as "reason" is a singular countable noun and always needs a determiner. Numbers 1-3 are correct, although for 2 and 3, it might be more natural to contract "is not" rather than "there is": 2b. There isn't a reason for you to be here. 3b. There isn't any reason for you to be here.

All are correct except #4. It’s because there is no indefinite article (a/an) before ‘reason.’