In order to combine fractions, they need to have the same denominator.
To figure out what value to make the denominator, find the least common denominator of 4 and 5:
4: 4, 8, 12, 16, 20
5: 5, 10, 15, 20
The LCD is 20, so multiply 4/5 by 4/4 and -1/4 by 5/5 to make the denominator = 20

Now combine the fractions
