The term 'SING SINGLY' can be solved by the word 'SOLO', which is a four-letter word. The term 'SOLO' is used to describe an activity or performance done alone, without accompaniment or assistance from others. It is often used in music to refer to a single singer or instrumentalist performing without accompaniment. In other contexts, it can refer to any activity done alone, such as a SOLO flight or a SOLO journey.