Now that we have Mythics, interesting 4-piece bonuses (and even 3-piece ones) that are not simply stats or harder to obtain trial bonuses are suddenly catching my eye. Unfortunately, the 3 and 4-piece bonuses currently available to us have very limited uses. I will list the ones I can find below (please let me know if I missed any).
What are your ideas on desirable 3 or 4 item set bonuses to pair with your builds and 1-piece Mythic items? I guess, it makes sense to keep them reasonable, since Mythic items are seemingly supposed to be replacements for our more powerful 5-piece and 2-piece monster set bonuses. So, it doesn't really make sense for a 3 or 4-piece bonus to be super powerful in this circumstance. However, given that some sets already in the game have unique 3 or 4-piece set bonuses, I don't see why they can't make more.
Unique 4-piece bonuses already in game:
Bahraha's Curse (Jewels, Weapons, Light/Medium/Heavy armor - Overland - Hew's Bane)
Set bonus
(2 items) Adds 1206 Maximum Health
(3 items) Adds 1096 Maximum Magicka
(4 items) Adds 1096 Maximum Stamina, Reduces your damage taken from environmental traps by 40%.
(5 items) When you deal damage, you have a 25% chance to create desecrated ground for 5 seconds, which reduces the Movement Speed of enemies within by 70%, damages them for 860 Magic Damage every 1 second, and heals you for 100% of the damage done.
Bastion of the Heartland (Jewels, Weapons, Heavy armor - Cyrodiil - Vendor)
Set bonus
(2 items) Adds 1206 Maximum Health
(3 items) Adds 4% Healing Taken
(4 items) Reduces your damage taken from Players by 5%.
(5 items) Reduces your damage taken from Siege Weapons and Player area of effect abilities by 20%.
Shield of the Valiant (Jewels, Weapons, Medium armor - Cyrodiil - Vendor)
Set bonus
(2 items) Adds 1206 Maximum Health
(3 items) Adds 4% Healing Taken
(4 items) Reduces your damage taken from Players by 5%.
(5 items) Reduces the cost of Break Free by 40%.
Syvarra's Scales (Jewels, Weapons, Light/Medium/Heavy armor - Overland - Hews Bane)
Set bonus
(2 items) Adds 1206 Maximum Health
(3 items) Adds 1096 Maximum Stamina
(4 items) Adds 1096 Maximum Magicka, Reduces your damage taken from Guards by 20%.
(5 items) When you deal damage, you have a 5% chance to cause a burst of lamia poison that deals 967 Poison Damage in a 5 meter radius and an additional 5802 Poison Damage over 6 seconds to all enemies hit. This effect can occur once every 6.5 seconds.
Vesture of Darloc Brae (Jewels, Weapons, Medium armor - Overland - Elsweyr)
Set bonus
(2 items) Adds 129 Magicka Recovery
(3 items) Adds 129 Stamina Recovery
(4 items) Reduces the radius you can be detected while Sneaking by 2 meters. Reduces the cost of Sneak by 10%.
(5 items) While crouching and not blocking, you restore 670 Magicka and Stamina and heal for 670 Health every second.
Way of Air (Jewels, Weapons, Medium armor - Overland - Craglorn)
Set bonus
(2 items) Adds 129 Stamina Recovery
(3 items) Adds 129 Stamina Recovery
(4 items) Increases the radius you can detect Sneaking enemies by 2 meters.
(5 items) When you use Roll Dodge, your Weapon Damage and Spell Damage is increased by 344 for 6 seconds.
Unique 3-piece bonuses already in game:
Blessing of the Potentates (Jewels, Weapons - Cyrodiil Vendor)
Set bonus
(2 items) Reduces your damage taken from Players by 5%.
(3 items) Reduces the cost of your Ultimate abilities by 15%.
Eagle Eye (Jewels, Weapons - Cyrodiil Vendor)
Set bonus
(2 items) Adds 833 Weapon Critical
(3 items) Adds 1829 Weapon Critical to your ranged direct damage abilities.
Night Terror (Jewels, Weapons, Medium armor - Overland - Stormhaven)
Set bonus
(2 items) Adds 1096 Maximum Stamina
(3 items) Reduces the radius you can be detected while Sneaking by 2 meters. Reduces the cost of Sneak by 27%.
(4 items) Adds 1206 Maximum Health
(5 items) When you take melee damage, you deal 2000 Poison Damage to the attacker. This effect can occur once every 1 second.
Vengeance Leech (Jewels, Weapons - Cyrodiil - Vendor)
Set bonus
(2 items) Adds 1206 Maximum Health
(3 items) When you kill an enemy, you heal for 2165 Health and restore 2165 Magicka and Stamina.
Wrath of the Imperium (Jewels, Weapons - Cyrodiil - Vendor)
Set bonus
(2 items) Adds 833 Spell Critical
(3 items) Adds 1829 Spell Critical to your ranged direct damage abilities.