That’s because the 615i Duo3 is limited to 180 watts per side. So around 11 amps maximum is about right for 4S.

The 4010 Duo, running both sides at the max of 40 amps into 4S, will run 1344 watts output maximum. Figure on about 1500 watts or so from the supplies, as the charger consumes some power.

The two supplies in series you mentioned would be just about perfect. Again assuming the 120V mains power is available. These big chargers are pushing if not exceeding the limits of a typical outlet-circuit.

Oh, kudos for realizing you have 74 amp 910 watt supplies on 110-120V mains power. Some don’t realize 220-240V power is required to get the 106 amp 1300 watt rating.