四sei3仔zai2

English

1. adult's videos; pornography
2. uncensored pornography; literally a "category four" film ("category three" is the highest classification for adults films in Hong Kong)
3. No.4 heroin

粵語 — Cantonese

1. 冇打格仔嘅鹹片；香港嘅電影分級制度最高係三級，「四仔」即係比三級片更色情、冇通過審查嘅電影（量詞：套／齣）
2. 四號海洛英，係一種毒品

Examples

• 喺香港賣四仔係犯法㗎。

  It is illegal to sell uncensored pornography in Hong Kong. ----

• 喺香港賣四仔係犯法㗎。

  It is illegal to sell heroin in Hong Kong.