A four-wheel steering central transmission device includes an input shaft extending through a front wall of a housing and having an input rotary disc formed with a first slide groove. A turning output unit includes an output shaft extending through a rear wall of the housing and having an output rotary disc formed with a second slide groove. A transmission unit disposed between the input and output shafts includes a hollow adjustment block disposed adjustably within the housing and movable leftward or rightward, and a transmission member movable within the adjustment block and having slide pieces inserted respectively and slidably into the first and second slide grooves.