So are Zed, Garen, Vi, and Sona actually dead?

It's like a 99% chance they're alive. It was all just to boost hype for Jhin, fortunately.

Riot's lore team is far capable enough to not make such a stupid mistake like killing off four champions to introduce one.

Rito if you dare let my beloved Zeddie die.... I`m voting for Trump.

No

Probably Jhin shot them because they were ugly, and he wants to make them beautiful or some shit lol.

No. At most, it's a part of Jhin's delusions. In all likelihood, it was simply a matter of marketing, as he has no interactions with Garen or Vi.