Which of the following sentences is correctly written?

We found a lot of earthworms lying on the sidewalk, when the rain ended.
We found a lot of earthworms lying on the sidewalk. When the rain ended.
When the rain ended we found a lot of earthworms lying on the sidewalk.
When the rain ended, we found a lot of earthworms lying on the sidewalk.

Question

Which of the following sentences is correctly written?

    We found a lot of earthworms lying on the sidewalk, when the rain ended.
    We found a lot of earthworms lying on the sidewalk. When the rain ended.
    When the rain ended we found a lot of earthworms lying on the sidewalk.
    When the rain ended, we found a lot of earthworms lying on the sidewalk.

anonymous · Answer

I am pretty sure it's When the rain ended, we found a lot of earthworms lying on the sidewalk.-the last one