The term 'GAS IN A DINER SIGN' can be solved by the 4-letter word 'NEON'. NEON is a type of gas that is used in many signs, including those found in diners. It is a noble gas that is colorless, odorless, and non-toxic. When an electrical current is passed through it, it emits a bright light that can be seen from far away. This makes it ideal for use in signs, as it is easily visible and can be used to attract attention.