Stella can move 1 box for every 4 boxes Michael moves.
She moves 10 boxes, so that must mean Michael moved 40 boxes (since 4*10 = 40).
Put another way, the ratio 1:4 bumps up to the equivalent form 10:40 after multiplying both parts by 10. The first value of each ratio is the amount of boxes Stella moves, and the second part is what Michael moves.