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This is actually completely incorrect - A mage in our guild got his 4p the other day and upon testing it, his 4stack AB cost was reduced by 600 some mana (exactly 10% of the cost of a 4 stack). Arcane power only increases the cost of AB by 10% of the Base spells cost (10% of 870(a 0 stacked ab) = 87). So a normally 4 stacked AB with arcane power up costs 6177 mana instead of 6090 according to my tooltip and tests done on a dummy.
However, once you add in the 4p, that base 87 mana increase is REMOVED and then the final cost of arcane blast is reduced by 10%. So at a 4 stack AB costing 6090 Mana, the t12 4set makes it only cost 5400 while arcane power is active.
This is very good news! I'm glad they changed it.