The standard way to say is:
みちこさんは(大学)四年生ですか?
みちこさんは(だいがく)よんねんせいですか?
In real campus life, many students also would say in this way,
みちこさんは四回生ですか?
みちこさんはよんかいせいですか?
(Being a 四回生, michiko might be a third year student if she remains in the same grade for two years. She even might be a first year student in fact! haha.)