In general, graduate students may take any course including and above the 4000 level. 4000 level courses tend to be undergraduate senior seminars that are also open to graduate students, 5000 and 6000 level courses are general introductory courses directed specifically at graduate students, 7000 and 8000 level courses are advanced colloquia on specialized topics, and 9000 level courses are advanced colloquia that emphasize reading primary sources (often in a foreign language). Language courses are open to graduate students regardless of number, and 3000 level history courses are an exception that remain open to graduate students.
A complete listing of all the courses offered by the department is at the Directory of Classes. This is distinct from and more complete than the various undergraduate bulletins.