北管細曲中有四套以「思」字為開始的大小牌曲目，分別為〈思凡〉、〈思夫〉、〈思秋〉、〈思潘〉等，這四套曲目不但常見於細曲抄本當中，亦成為用來論斷某位北管先生是否夠格的標準之一。「四思」之所以廣被流傳，除了曲調變化較多、旋律優美外，在不逾越傳統禮教的規範下，透過音樂在保守社會中傳唱著無法明說的男女思念或是閨房情事，不但成為酒樓茶肆裡的流行曲調，亦是北管子弟夜晚時分的另類娛樂。就樂曲本身而言，從「四思」的曲牌組成結構，可更加清楚看出大小牌的身份淵源，有來自明清小曲系統而自行連結成套的小牌，亦有與戲曲或南北曲牌關係密切的大牌曲調，此二者在北管的細曲中融為一類獨特的曲類。

In the repertoire of the large and small labeled tunes of beiguan refined songs, there are four vocal suites called the “four s.-suites” (四思) because their titles are all begun with a Chinese character “s.” (思): “S.-fan”, “S.-fu”, “S.-qiu”, and “S.-pan”. They feature the artistic and sophisticate beauty of beiguan refined songs. Therefore they have not only been commonly included in beiguan manuscripts, but also been used to be one of the criteria for evaluating beiguan masters.
The “four s.-suites” was widely spread among beiguan practitioners under two reasons: one was because the elegance and diversity of their melodies, another was because of the music and lyrics which were able to express love and romance and to cross the boundary of decorum restricted by a feudal society. Therefore the four suites were popular in teahouses and pubs and became a special entertainment for beiguan zidi (amateur music practitioners) at night.
By examining the structures of the “four s.-suites”, we can clearly understand the sources of the large and small labeled tunes in beiguan refined songs. Some of them are small labeled tunes originated from Mingqing xiaoqu (folk tunes in the Ming and Qing dynasties), the others are the large labeled tunes related to Chinese theatre music or nanqu and beiqu (southern and northern songs). The large and small labeled tunes have been combined together and become a special category that feature beiguan refined songs.