Explanation:

- The Bhopal gas tragedy is recognized as the deadliest industrial disaster in history.

- A staggering 42 tonnes of methyl isocyanate leaked from the Union Carbide factory.

- A toxic cloud enveloped the area, causing widespread harm.

- The disaster resulted in both immediate and long-term fatalities.

Let's look at the options for blank 4:

1. Merciless: Describing something harsh or severe; not typically used for gases.

2. Deathly: Refers to something causing death; suitable for describing a deadly gas.

3. Incurable: Suggests something that cannot be cured; doesn’t convey immediate danger.

4. Abominable: Means something morally bad or unpleasant; not specific to gas effects.

Deathly is the best fit here as it directly refers to causing death.