Ask her to try hand squirting before she puts bub on. And to physically relax while feeding.
SHe might also find that feeding a bit sooner than she thinks she has to gives her some more wriggle room before bub gets grumpy.

Stress can slow let-down, and 3-4 month olds are impatient critters. It's important to remember that if bub is growing and weeing and pooing then enough milk is going in!