Ye Olde Inn

As much as I like the potential for a hero to lose Borin's Armor and Orc's Bane in this quest; I feel the Spirit Riders are overall irrelevant to the completion of this quest. The stairway is in the middle room through a secret door, which the heroes can potentially find without even encountering the Spirit Riders.

These Spirit Riders, to me, are likened to the 4 Horsemen of the Apocalypse: Death, Famine, Pestilence and War. I'd really like to make it mandatory for the heroes to fight at least some of them. So whenever... if ever, my group gets to this quest I'd like to make some editions:
First off, there will not be a secret door leading to the center room, I intend to us the exit door, put it around the corner, leading to the middle roo, but have it locked. The key will be inside one of the Spirit Riders' coffins.

When my group plays treasure chests need to be searched separate in rooms. Rooms are granted a single search, unless there's a Treasure chest, which always has a specific item or amount of gold in it. I'd like to apply the same additional search to coffins. In fact, the introduction to the coffins in the quest book reads: "The 4 coffins may be used as tombs. They may contain an undead creature and/or a Treasure. They may also have traps on them."
In the case of the Cold Halls I'm wanting to hide the key to the exit door (which leads to the room with the Spiral Staircase) in one of them. But I'm thinking each coffin should have something of note in them.

One coffin will have the key. I'm thinking filling another one with poisonous spiders that spread across the room once it's opened and anyone in the room at Zargon's turn rolls combat dice for damage (might as well since Venom Antidotes are in the Alchemist shop). I'm also thinking of adding another Potion of Mystery, much like the one from Quest 7 Stone Hunter/Lost Wizard in the game system, but I'm not sure what the effects should be, any suggestions?

If you want to make a reference to the four riders of the Apocalypse, you could give each a special effect or ability, or do something within the respective room, that hints at their specialty.

Or put their dead horses in with them, like some of the Kings and Khans of old did. And when you open the door to the room, the skeletal horses attack too.

I was more so looking for ideas to subtly reference the 4 Horsemen by certain Treasure searches of each coffin.
Death, Famine, Pestilence and War
A Potion of Mystery found in Death's coffin, when drank, reduces the heroes body points to 1, but all undead during that quest (whichever quest he drinks it) will not attack that hero.
Search the coffin of Pestilence and a colony of venomous spiders spread across the room attacking all heroes in the room for 3 combat dice.
War's coffin could have a battle Axe or Broadsword, something to symbolize war.
What could I use to symbolize Famine?

Take away all remaining water spells.