Literally: mosquito – horsefly – run – cow
Alternately: Something powerful being defeated by something weak; the great overthrown by the small. Literally, a cow being driven to run away by the onslaught of tiny biting flies.
Alternatively, this can refer to large events, especially terrible catastrophes, developing out of small matters; a destructive stampede being caused by mere mosquito-bites.
Notes: This phrase comes to us from the Garden of Stories (Chinese 說苑 = Shuo Yuan, Japanese 『説苑』 = Zeien), a two-millennia-old anthology.
Apparently this four-character compound can be read in sentence form as 蚊虻牛を走らす (bunbou ushi wo hashirasu), and is considered a contraction of the phrase 蚊虻牛羊を走らす (bunbou gyuuyou wo hashirasu), adding sheep to the list of large animals that the flies send running.