Sarah's reasoning sounds good to start. She rounded the available bags and their content, so the result, 4000 bracelets, should be really easy to reach.
The exact numbers are 214 bags * 22 bracelets / bag = 4708 bracelets; so as you see, Sarah took care of having room to loose some beads and yet the beads will be enough to make 4000 bracelets.
Of course, if you want to be sure, you should ask Sarah some questions. For example:
1) What proportion of beads are normally deffective?
2) What proportion of beads are normally lost when crafting?
So, if Sarah did not take into account those proportions she migth be overestimating the number of bracelets they can make.