So I found a revision history of mage T17 set bonuses which go all the way back to WoD beta which I found pretty interesting and decided to share (these are ordered from earliest to latest, by spec). If no 2P or 4P is listed for a build, then nothing was changed for the other:
Frost
Build 18379
2P - Increases the damage of Frostbolt by 10%.
4P - When you active icy Veins, you also gain Arctic Power. Arctic Power increases your Multistrike damage by 10% for 20 sec.
Build 18443
2P - When you active icy Veins, you also gain Arctic Power. Arctic Power increases your Multistrike damage by 10% for 20 sec.
4P - Frostbolt critical strikes have a chance to strike the target with an Ice Shard, dealing (100% of Spell power) extra damage to the target.
Build 18471
2P - When you active icy Veins, you also gain Arctic Power. Arctic Power increases your Multistrike damage by 33% for 20 sec.
4P - Frostbolt has a 10% chance to surround the target with icy spikes that all converge, dealing (100% of Spell power) extra damage to the target.
Build 18522
2P - While Frozen Orb is active, you permanently have the Fingers of Frost effect.
4P - Each Ice Lance cast while Frozen Orb is active generates one stack of Ice Shards. When Frozen Orb ends, Ice Shards reduces the cast time of Frostbolt by 25% and increases the damage of Frostbolt by 30%, consuming one stack each cast.
Build 18537
4P - Each Ice Lance cast while Frozen Orb is active generates one stack of Ice Shards. When Frozen Orb ends, each application of Ice Shards reduces the cast time of Frostbolt by 5% and increases the damage of Frostbolt by 5% for 10 sec.
Build 18663
4P - Each Ice Lance cast while Frozen Orb is active reduces the cast time of Frostbolt by 5% and increases the damage of Frostbolt by 5% for 10 sec. Stacks up to 10 times.
Build 18950
2P - Whle Frozen Orb is active, you gain Fingers of Frost every 2.5 sec.
4P - Each Ice Lance cast while Frozen Orb is active reduces the cast time of Frostbolt by 2% and increases the damage of Frostbolt by 2% for 10 sec. Stacks up to 10 times.
Fire
Build 18379
2P - Reduces the cooldown of Inferno Blast by 2 sec.
4P - Increases the damage of Pyroblast by 10%.
Build 18443
2P - Casting Inferno Blast grants you the Firestarter effect. Firestarter reduces the cast time of your next fireball by 20%.
4P - When Hot Streak activates, you have a 10% chance to gain the Pyromaniac effect. Pyromaniac causes all Pyroblasts to have no cast time and be guaranteed critical strikes.
Build 18471
2P - Casting Inferno Blast grants you the Firestarter effect. Firestarter reduces the cast time of your next fireball by 25%.
4P - When Hot Streak activates, you have a 100% chance to gain the Pyromaniac effect. Pyromaniac causes all Pyroblasts to have no cast time and be guaranteed critical strikes.
Build 18522
2P - Inferno Blast now has 2 charges.
Build 18537
4P - When Hot Streak activates, you have a 100% chance to cause all Pyroblasts to have no cast time and be guaranteed critical strikes for 6 sec.
Build 18594
4P - When Hot Streak activates, you have a 100% chance to cause all Pyroblasts to have no cast time and be guaranteed critical strikes for 6 sec. (Approximately 1.50 procs per minute)
Build 18850
4P - When Pyroblast! activates, you have a 100% chance to cause all Pyroblasts to have no cast time and be guaranteed critical strikes for 6 sec. (Approximately 1.50 procs per minute)
Build 18950
4P - When Pyroblast! activates, you have a 100% chance to cause all Pyroblasts to have no cast time and be guaranteed critical strikes for 4 sec. (Approximately 1.00 procs per minute)
Build 19551
4P - When Pyroblast! activates, you have a 100% chance to cause all Pyroblasts to have no cast time and be guaranteed critical strikes for 4 sec. (Approximately 0.70 procs per minute)
Arcane
Build 18379
2P - Increases the damage of Arcane Blast by 10%.
4P - Increases the damage bonus of Arcane Power by 10%.
Build 18443
2P - After using Evocation, you gain the Arcane Affinity. Arcane Affinity reduces the cast time and mana cost of Arcane Blast by 10%.
4P - When you gain an Arcane Charge, you have a chance to gain the Arcane Instability effect. Arcane Instability increases the number of missiles your next Arcane Missiles will fire by 10.
Build 18537
2P - After using Evocation, the cast time and mana cost of Arcane Blast is reduced by 10%.
4P - When you gain an Arcane Charge, you have a chance to increase the number of missiles your next Arcane Missiles will fire by 5.
Build 18594
2P - After using Evocation, the cast time and mana cost of Arcane Blast is reduced by 25% for 8 sec.
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Personally I think the Frost 2P iteration where each Ice Lance gives you one empowered Frostbolt was the best idea, it would be a lot better than the current bonus.