In order to pour drinks prettily!
Literally: male – butterfly – female – butterfly
Alternately: This noun phrase describes a pair of origami “butterfly” shapes sometimes attached to the decanters used for pouring sake during traditional Japanese wedding ceremony. By extension, the child whose job it apparently was to pour the drinks. Alternately, the prosaic reading of “male and female butterflies.”
Notes: It was harder than I had thought to find a four-character compound starting with を. Most Japanese words that classically began with を use the Japanese reading, while compounds tend to use Chinese readings – for example, 鬼 used to be をに, but in yojijukugo it tends to be read as き (ki). Fortunately both 雄 (“male”) and 男 (“man”) can be read as を, so here we are.