History of The Fourth Horseman

The Fourth Horseman is an Arc Exotic Shotgun that returned from the original Destiny. It is a fixed-roll weapon, so every copy has the same perks.

Overview

• Intrinsic trait – Thunderer: greatly increased rate of fire and full-auto fire. Final blows auto-load ammo from reserves, and defeating powerful targets fully reloads the weapon.
• Exotic perk – Broadside: each successive shot deals more damage but has more spread. This replaces the original game's "Return to Sender" bonus-ammo perk.
• Archetype: Shotgun (Arc).

Acquisition

• Season of the Worthy quest: it was first earned through a 10-step Exotic quest in Season of the Worthy: gather intel on Mars, collect encryption signals in the EDZ, and finish by clearing the Legendary Lost Sector The Quarry on the Sunken Isles (per Game Rant).
• Exotic Archive: it now sits in the Exotic Archive at the Tower. Search results from Mobalytics/Gamepur-type guides list the price as 1 Exotic Cipher, 125,000 Glimmer, 200 Legendary Shards and 1 Ascendant Shard. Costs may have changed, so check the Archive.

Catalyst

The weapon has a catalyst (The Fourth Horseman Catalyst); guides summarise it as adding a round to the magazine and improving reload speed. Details on unlocking it were not confirmed in the sources opened, so see that page.