Explanation:

Let speed of boat and current be x and y.
According to question,
x +y =20 / 2=10 ….. (i)
x − y =4 / 2=2 ……… (ii)
Adding (i) and (ii) we get
2x=12
=> x = 6
From (i)
y =10−x=10−6=4
Thus speed of current be 4 km/hr
Hence option (4)