The current is defined as the amount of charge that flows through the circuit in a certain time interval:
[tex]I= \frac{Q}{\Delta t} [/tex]
where
I is the current
Q is the charge
[tex]\Delta t[/tex] is the time interval
In our problem, the current is [tex]I=1.5 mA=1.5 \cdot 10^{-3} A[/tex] and it runs for a time of
[tex]\Delta t= 4.0 h =14400 s[/tex]
so we can use the previous equation to find the charge transferred from one terminal to the other terminal of the battery during this time:
[tex]Q=I \Delta t= (1.5 \cdot 10^{-3} A)(14400 s)=21.6 C[/tex]
