The manga is a collection of 4 oneshots, published between 1989 and 1995 in Shueisha's Weekly Shounen Jump magazine:

ZETMAN(特別編集オータムスペシャル); Shounen Jump 1994, Special Autumn issue
Jin Kurono's father is dead, so he has been raised by his mother, who taught him the sense of honor and justice. But she died killed by a car, which ran away after the accident, leaving a scar in Jin's heart. Now, he's 23 years old, and he is the programmer of a new game: Zetman. Upset with the fact that there is no justice in the real world, he turns into his own superhero Zetman, to fight against crime, but with hard methods. To advise him and support him, he will always have his friend Sachiko Shirai.

Shin-no-Shin: Ai to Nikushimi no Time Slip (SHIN-NO-SHIN 愛と憎しみのタイムスリップ); Shounen Jump 1989 #31

Woman in the Man: Otoko no Naka no Onna (WOMAN IN THE MAN -男の中の女-); Shounen Jump 1993 #5+6
Tsuyomaru is a girlish boy while Hazumi is a tomboyish girl. Tsuyomaru is challenged to a fight by a student of another karate school, and while using a training machine operated by Hazumi, something goes wrong and they switch bodies...

Shadow Lady (pilot); Shounen Jump 1995 #3+4; Also included in volume 3 of Shadow Lady