mAh rating is total, not combined. So any battery made with 4000mAh cells will be 4000mAh.
Also, a 4000mAh battery would be made with large cells...minis don't go that high. Max for minis is about 1500mAh. Look for 2/3A cells to make a stick pack for an AK, not Sub-C (SC) cells. Get 18 gauge wiring.
Your stock gun would have a male mini-Tamiya connector. So you'd need a female airsoft mini-Tamiya on the battery. But I'd recommend changing both to deans.
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