In comparing two fractions, a/b and c/d, one can use cross multiplication.
If ad > bc, then a/b > c/d. If bc > ad, then c/d > a/b. If bc = ad, then a/b = c/d
Applying that concept to your question, we cross multiply 4/8 and 6/8
Is 8*4 > 8*6? No.
Is 8*6 > 8*4? Yes! Since 6/8 > 4/8, then Joe bought mroe grapes than Peter.