Electric vehicle batteries with a voltage of 4V are usually already at a loss. Electric vehicles are currently composed of 12V batteries connected in series. A fully charged 12V battery can reach around 13.8V. A voltage below 9V is considered overdischarging, while a battery with only 4V voltage is clearly a serious form of overdischarging.
Power depletion has a serious impact on battery life. After the battery of an electric vehicle loses power, its lifespan will be reduced by 80%. If the battery is in a depleted state for a long time, sulfurization will occur, resulting in a decrease in the effective substance of the electrode plate, a decrease in discharge capacity, and a significant reduction in service life.
You can try charging slowly with a pulse charger. A severely depleted battery with an open circuit voltage below 10V can be slowly charged using a pulse charger.