The d4 is a platonic solid, the tetrahedron. Gamers sometimes colloquially refer to 4 sided dice as caltdrops and most veteran gamers have accidentally stepped on the sharp top edge of these dice at least once. Non-gamers often call them pyramid dice.
d4 dice have three numbers on each of the four faces, and you read them by looking on any side for the number that is right-side up. On older d4 dice, including the first ones made for D&D, the correct number was in the middle of the bottom edge of the dice. Modern 4 sided dice place the number in the corners rather than along the sides, and you read the result from the top corner.
4-sided dice similar to the modern d4 are actually the oldest dice ever discovered in archeological digs. The Royal Game of Ur used d4s and one was uncovered in an ancient dig site and dated to over 5,000 years old.