A trademark signal is a LOGO or symbol that is used to identify a product or service. It is usually registered with the government and is protected by law from being used by other companies. The term 'trademark signal' can also refer to a sound, phrase, or design that is associated with a particular brand. The four-letter solution for this crossword is LOGO, which stands for 'logotype, ' a graphic representation of a company's name, initials, or trademark.