I’ve played a controll priest mirror today where the other priest (my opponent) plays Rivendare, Warrider.
Rivendare, Warrider died und activates the deathreattle to shuffe the 3 other horsemen into my opponents deck.
He played 2 of the shuffed riders and killed them self so they trigger their deathrattle normal.
I played False Disciple (from Titans Expansion) and add with it Chameleos (from Ungoro Expansion) to my hand.
Next round it converts to the last (of the 3 shuffed) riders my oppent hold on his hand.
I played Chameleos so that i get the last missing rider on my deck side and destroy it - but nothing happens and i don’t win the game with this interaction (this should be a bug).
My opponent do the same with his version of the card the turn after and wins the game.
Rivendare and the other 3 riders must die on the same side of the field.
Rivendare and 2 of the riders died on your opponents side and then 1 of the riders died on your side.
Only 1 rider died on your side which is not enough to trigger the deathrattle and then your opponent played the last rider and won because 4 riders died on his side.
What Joachim says.
This is indicated in the text on the card:
" Deathrattle: If you [emphasis mine] had all four Horsemen die this game, destroy the enemy hero."
Thank you for the actual Text in the English version of this card.
I found only an other (older?) version of the English card with was identical in translation to the actual German version.
Then it’s a German localisation problem which was still fixed in the English Version:
The three aditional riders said translated to English in the German Version (which is also found under https://hearthstone.blizzard.com/de-de/cards/86725-rivendare-warrider):
Lifesteal|Taunt|Rush
Deathrattle: Destroy the openent hero if in this game(!) all 4 riders died.
The German Version don’t tell you that they must be on your side it said died in this game instead.