Let the number be n.
Four fifths of the number is 4n/5.
Three fourths of the number is 3n/4.
Four fifths of a number is more than three fourths of the number by 4.
4n/5 = 3n/4 + 4
20 * 4n/5 = 20 * 3n/4 + 20 * 4
16n = 15n + 80
n = 80
Let's check.
4/5 of 80 = 4/5 * 80 = 64
3/4 of 80 = 3/4 * 80 = 60
64 is indeed 4 more than 60 showing that 4/5 of 80 is 4 more than 3/4 of 80, so 80 is correct.
Answer: 80