Good advice. Maybe have a fire extinguisher handy also.I'd just remove the battery from the vehicle and try the repair outside of the vehicle.
But yeah... sounds like the battery got shorted with no fuse to interrupt. Surprised you didn't hear anything honestly.
EDIT— Just for some added details: AGM batteries are considered to be fully discharged around 10.5 - 10.7 volts. When it's below that voltage it's considered "deeply discharged" and typically permanent damage occurs. And you said you were around 4 volts... so.... it's probably safe to assume the battery is dead or severely damaged. Hence why you want to keep it away from the Bronco.
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