To convert pounds to ounces, you can use the conversion factor that 1 pound is equal to 16 ounces.
So, to find out how many ounces are in 4 pounds:
\[ 4 \text{ pounds} \times 16 \text{ ounces/pound} = 64 \text{ ounces} \]
Therefore, a bag of apples that weighs 4 pounds weighs 64 ounces.