This company's clients are spread across the whole country.
Tips
history
The component parts go back to antiquity — 五湖 appears in 《周礼》, 四海 in 《论语》 (四海之内,皆兄弟也, 'within the four seas, all men are brothers'). Fused as a single idiom by the Tang, it became famous in modern times through Mao's eulogy 《为人民服务》.
culture
The 'five lakes' originally meant specific lakes around Taihu; 'four seas' reflected an old worldview where China was bordered by oceans on all four sides. The idiom long ago shed the geography and just means 'everywhere.'