Let the height of first brother be x. As the difference in heights of the first 3 brothers is 2,
heights of 2nd and 3rd would be x+2 and x+4 respectively.
Also difference between third and forth brother’s heights is six inches, thus the height of fourth one would be x+10.
Average height of 4 = 74 inches => sum of heights = 74*4 = 296
=> x+x+2+x+4+x+10 = 296
=> 4x+16 = 296 = > 4x = 280 => x = 70
Thus the heights would be 70, 72, 74 and 80 respectively.