Explanation:

Correct option 4: went

Let’s examine the sentence with Blank 3 in context:

"...while the boys (3) ______ to school..."

We are talking about a past habit or general action in the past, as seen from the first part:

"Long ago in China..."

So, we need a past tense verb.

Option analysis:
going – Present participle; incorrect in this context.

goes – Simple present (singular); doesn't match the plural subject "boys".

go – Present tense (plural); still inappropriate because the context is past.

went –  Simple past tense; correctly matches the time frame