The series takes place a few months after the 3rd installment.
Chan Siu Sang and Wai Ying Zi have been dating for a while and seem to be in a stable relationship. Everything seems to be looking up for Ying Zi now that she's in a stable relationship and finally got a promotion at work, but the only that she and Siu Sang seem to fight about is marriage. He wants to rush into marriage but she wants to put it off for a few years to focus on her career since it is flourishing. Their relationship is further complicated when Siu Sang becomes jealous of Ying Zi's new handsome co-worker Ken, Mad that Siu Sang does not trust her, the two decide to break up. Ying Zi soon starts dating Ken since the two grew close while working together and Siu Sang begins dating Sunnie who works at the forensic department after being pursued by her.
Having gone through so many tragedies in the past Chan Sam Yuen finally decides to focus on her family as top priority. She and Ching Fung have patched up their marriage even though it is still not perfect. With a difficult pregnancy Sum Yuen is no longer focus on her career. All seems well with her and Fung when she gives birth to a son until Fung gets entangled with female triad boss Chiang Tin Wing and her husband Sing Jin Choi. Fung is in charge of the case of Wing and her crime empire. While investigating Wing, she takes a liking to Fung and drugs him so she can have her ways with him. She uses their one night of passion to blackmail Fung into continuing their affair. Choi finds out about the affair between his wife and Fung, enraged with his wife's infidelity he accidentally kills her and frames Fung for her murder. With evidence to clear Fung of the crime Choi decides to kill Fung as revenge for ruining his marriage.
- Native Title: 陀槍師姐IV
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