The distinction here is between vagueness and precision. 1 and 4 are vague, 2 and 3 precise.
Option 1, using the preterite / simple past / past indefinite tense is vague. It could mean "I saw the cat [which had been] struck by a car" or "I saw the cat [as it was being] struck by a car". The idea in parentheses only covers one of these, but we don't have sufficient information to tell from the sentence which was intended.
2 (present participle) and 3 (past perfect) are precise, and the ideas in parentheses are accurate.
4 is also vague. The imperfect past progressive / past continuous tense of "was struck" doesn't help us, and we would want some additional context, for example :
I was walking along the street and I saw that the cat was struck by a
car (suggests seeing the event)
I was examining the pet and I saw that the cat was struck by a car
(suggests seeing consequences)