the number four; complex form used for financial purposes is 肆 sì

the string of a musical instrument; (by extension) stringed instruments or music; (bound form) to play a stringed instrument; to bend or curve something

(bound form) a general name for musical instruments, especially stringed ones like the 钢琴 gāng​qín, 提琴 tí​qín, or 胡琴 hú​qín; also used for certain keyboard and reed instruments