Grades four, five, and six in a certain school organized a cultural performance with a total of 18 performances. If each grade performs at least 4 programs, how many different situations are there in the number of performances for these three grades?
某学校四、五、六三个年级组织了一场文艺演出，共演出18个节目，如果每个年级至少演出4个节目，那么，这三个年级演出节目数的所有不同情况共有多少种？
<result>4</result>