In order to find the correct answer you need to have the same denominator. Change them to improper fractions:
4 5/7= 33/7
3 3/4= 15/4
To get the same denominator find the lcm.
You were right with that, 28.
To even this out multiply 33 by 4, 132. 132/28
15*7= 105. 105/28
132/28 + 105/28 = 237/28
237 divided by 28 = 8 13/28