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I believe the iPad 4 charger puts out 2.4 Amps and the Ipad 3 charger puts out 2.1 Amps.

I'd have to double check when I get home.
 
They both have the sames sized battery, and so will charge in the same time using the same charger. Since the 4 ships with the 12W charger, it charges a little quicker than the 3 with its 10W charger. Get the 12W charger for the 3 and it will charge in the same time.
 
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