What is All Fours?

Legal Definition
All Fours, also known as High-Low-Jack or Seven Up, is an English tavern trick-taking card game that was popular as a gambling game until the end of the 19th century. It is the eponymous and earliest recorded game in a family that flourished most in 19th century North America, notable other members being Auction Pitch, Pedro and Cinch, which competed against Poker and Euchre. Nowadays the original game is especially popular in Trinidad and Tobago, but a simpler variant has also survived in parts of England. Martinique is also accredited with the creation of all fours by coming up with the popular 'Two in Tobago ' rule.

Each player is dealt six cards. In trick play, players are allowed to trump instead of following suit. The title refers to the possibility of winning four game points by being dealt both the highest and the lowest trump in play, capturing the Jack of trumps and winning the greatest number of card-points.
-- Wikipedia
Legal Definition
This is a metaphorical expression, to signify that a case agrees in all its circumstances with another case; it goes as it were upon its four legs, as an animal does.
-- Bouviers Law Dictionary
Legal Definition
Two cases or decisions which are alike in all material respects, and precisely similar in all the circumstances affecting their determination, are said to be or to run on "all fours."
-- Black's Law Dictionary