And it's weight, so the capacity will be a half smaller than the one in T4i.
not at all!
Rebel T4i/EOS 650D: LP-E8, 7.2V, 1120mAh = 8,1 Wh
EOS-M: LP-E12, 7.2V, 875mAh = 6.3 Wh
= 78% of the LP-E8
Even if the shutter mechanism and fps are identical between T4i and EOS-M, power consumption of the EOS-M will be lower as no mirror needs to be flipped all he time. This will be partially offset by the need to constantly use the LCD as viewfinder to compose images. Overall, I would expect a fairly similar number of shots per charge from both cameras.