Median is a middle value.
Arrange the numbers from smallest to largest.
The two middle numbers are 4 and 6. The median would be 4+6 = 10/2= 5
Replacing a 4 with a 9 the two middle numbers are now 6 and 8, making the median 6+8 = 14/2 = 7
The median increases by 7-5= 2
The answer is 2