Well base upgrades would apply as well. You wouldn't only get tier4 specials.
So tier4 caster weapon could be:
Base: Except +1, Exceptional Cha +2 (Concordant op)
Tier4: Exceptional Cha +4 (total +7 stackable charisma)
Epic Concordant Oposition.
Procrate doubles (8%, and provides double the spell points and temporary hitpoints as normal)
Could also modifier the lower tiers, as Epic weapons can have different properties. Especially ther tier2 specials (clickies)
Since caster nuking is leaning towards temporary effects for massive spell damage boost, some clickies should be change into epic damage boosts:
IE:
Fire2,
old = Delayed Blast fireball (Who cares)
new = Ultra Inferno IX, 3 per day.
Duration: 3 minuit
Effect: +105% damage to fire spells.
There could even be new tier4 combos, which combine old double shard bonuses, into new quad shard abilities.
Caster Example: Double Epic shards in Tier4, Combining a Epic Concordant op gem with an existing triple air (air guard item)
= Air of Oposition, Whenever you are struck by lightning damage, you have a 10% chance of regenerating 1000 spellpoints. This effect can only proc once per rest.
Melee Example:
Triple Fire Weapon combined with a Epic Magma Surge Shard:
Epic Inferno:
This weapon holds incredible firey power. Each time it deals damage, it has a small chance (0.5%) to invoke a devastating inferno!
Inferno is similar to the Black Abbots, engulfing the entire area (massive AOE) in a firey inferno. 500 fire damage every 1 second for 20 seconds. Also inposes a -50% runspeed and attack speed debuff on enemies, and prevents enemy spell casters from casting.
Monsters that are otherwise immune to curses are immune to this effect (all purple named, some red name). This effect can only proc once per rest.