This is a copy of the famous stone water basin at the Ryoan-ji Temple. The square in the middle is the Chinese character for "mouth" and the four characters written around it all have the element of "mouth" within them. Together, read clockwise starting from the top, they make up a Buddhist proverb which losely translates as "If someone poor is satisfied with what he has, he is rich (at heart), while someone rich, who is not satisfied with what he has, is poor (at heart)" There is quite a lot that can be expressed with just four Chinese characters, isn't it?