ans: 3,600 litters.

Let, the capacity of the cistern be x liters
The pipe can fill in 1 minute = 4 liters
The pipe can fill in 60 minute = 4 x 60 = 240 liters

Time to fill the whole cistern = $\frac{x}{240}$hrs.

so in $\frac{x}{240}$ hrs the pipe can fill = 1 cistern

In $\frac{x}{240}$ hrs the pipe can fill = $\frac{x}{240}$ part of the cistern
Again, the leak can empty in 1 hrs. $\frac{1}{6}$ part of the cistern.
And now, in 1 hr. they can empty $\frac{1}{10}$ part of the cistern
So we can from the following equation

=> $\frac{1}{6}$ – $\frac{1}{10}$ = $\frac{240}{x}$

=> $\frac{10-6 }{10\times 6}$ = $\frac{240}{x}$

=> 4x =240 x 60

=> x = $\frac{240\times 60}{4}$

x = 3600