How can the charging time of a cell phone battery be calculated?
You don't tell us what voltage is required to charge your 4 V battery so let's assume it's 5 V.

Linear regulator / charger


If we use a linear voltage regulator or charger we can drop the 10 V down to 5V and can draw 2 A from the power supply. Power into the battery will be 5V x 2A = 10 W. Power dissipated in the regulator will be 5V x 2A = 10 W. Efficiency will be 50%. 
Charge time would be \$ \frac {Ah~capacity}{current} = \frac {20Ah}{2A} = 10~h \$ if the battery could store all the energy. Since the battery gets warm during charging we know that some of the input power is being lost so 12 h may be a better estimate.


Buck charger

Using a buck voltage converter is more efficient and when reducing the voltage the current can be increased. Let's assume we could get one with 85% efficiency.

$$ V_O \cdot I_O = \eta V_I \cdot I_I $$ where \$ \eta \$ is the efficiency. From this we can work out the maximum output current is

$$ I_O = \eta \frac {V_I \cdot I_I}{V_O} = 0.85 \frac {10 \cdot 2}{5} = 3.4~A $$

Ideal charge time will be \$ \frac {Ah~capacity}{current} = \frac {20Ah}{3.4A} = 5.9~h \$ so say 7 hours allowing for heat losses.

I hope the formulae weren't too obscure. ;^)