I’ve reported this via Twitter many times without so much as a reply. The hint says it drops in legendary lost sectors. I’ve run over 300 of them solo and still no sign of it. I’ve completed every other catalyst and received every other exotic. The Fourth Horseman Catalyst is not dropping in legendary (or master) lost sectors.
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Hey there. Fourth Horseman's catalyst dropped from Season 10's 'Legendary' Lost Sector activity. It does not drop from the Legend/Master Lost Sectors that were reintroduced in Season 13. At the moment the catalyst does not drop from anything. Unfortunately the Collections contain a fair amount of inaccurate or outdated information on their drop sources. While I'd say that 'Legendary' and 'Legend' Lost Sectors are two different named activities, I can see why people would disagree and they should have changed the description. Hopefully the catalyst will return sooner rather than later.