A baker is filling equal-sized containers with sugar. Four pounds of sugar fill 1 1/3 containers. How many pounds fit in each container?

1/3 pound

This is messy to put into words. Hopefully someone else can clean it up for you.

1 1/3 divided by 4.

Make the 1 1/3 an improper fraction, = 4/3. It may help to think of the whole number, 4, as 4/1.
When dividing fractions, flip the bottom one and then multiply.

So 4/3 over 4/1 becomes
4/3 x 1/4 = 4/12 = 1/3