also: よんそくほこう
二足歩行 means bipedal walking (on two legs), while 四足歩行 means quadrupedal walking (on four legs).
Standard kanji spelling. The reading しそくほこう is the most common.
Alternate reading, less common but still used.
Compound of 四 (four), 足 (legs), and 歩行 (walking).