The height of the person and mountain, the radius of the earth, and the distance to the horizon formulate a relationship of Pythagorean theorem. 
Thus , hypotenuse = sum of the radius of the earth and the height of the person and mountain. 
C = 3959 mi + 4.3 mi 
 = 3963.3 mi 
C2 = A2 + B2 
B=√C2−A2 
 
B =√(3963.3 mi)2−(3959)2 
B =184.6 mi 
The distance to the horizon is 184.6 mi