Some people always sign the number 4 with the palm forward. For others, however, the palm faces back for the actual number 4 but the palm faces forward when used as a part of a longer number like 555-1234, as a time (e.g. four o'clock), and in certain other numeral-incorporating constructs.
Like other single-digit ASL numerals, this sign is incorporated into several other signs to indicate a four instances. Several of the derived terms below show that numeral incorporation.
This can also be used in classifier constructions to represent four upright individuals acting together.