"What did she think as she walked in the park?" vs. "What was she thinking as she walked in the park?"

Question

1. What did she think as she walked in the park?
2. What was she thinking as she walked in the park?

I wonder what is the possible difference in meaning between the two sentences.

I wonder also how the meaning would change if we add about:

3. What did she think about as she walked in the park?
4. What was she thinking about as she walked in the park?

Which sentence of the four would be best suited to ask for the contents of the woman's thoughts?

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(based on a sentence from a text I was proofreading at lang-8)

Answer 1

What was she thinking is an idiom that expresses surprise at someone's carelessness or thoughtlessness. So, Sentence 2 could be synonymous with Sentence 1, or Sentence 2 could be expressing surprise that she was walking in the park. (Perhaps the park is known to be a dangerous place.)

Adding the word about eliminates the ambiguity with the idiom. I would say that Questions 1, 3, and 4 are all pretty much synonymous: they inquire about the thoughts that were running through her mind as she walked in the park, although Questions 3 and 4 seem to ask about multiple thoughts, while Question 1 could be construed to imply that she was thinking about one thing at a certain point in time, and the writer is wondering about that one thing.

They are all fine in regards to a woman's thoughts. They would be fine for a man's thoughts, too – all you'd need to do is change the pronouns:

What did he think about as he walked in the park?