I have a doubt if 4 is right.
Someone told me that to choose between who and whom, count the verb and subject pairs. If all the verbs have been used with their respective subjects and there is no verb left without a subject, we should use "whom" as it is an objective case.
The 4th sentences falls under the same category. The pairs of subject and their respective verbs are- "He is the man", "I found responsible for the accident".
Is the above rule or advice that I have mentioned wrong?
That sounds like an unnecessarily complicated (and perhaps dubious) procedure to me. In a relative clause, there is a "gap." The simple way to decide between "who" and "whom" is to look at whether a subjective- or an objective-case pronoun is "needed" (in an ideal grammatical world) in the gap.
You quote "I found responsible for the accident" above as if it were a complete sentence. It isn't. It has a "gap": "I found __ responsible for the accident." Would you say,
*"[strike]
I found he responsible for the accident[/strike]"? No. You would use "him" (objective case). Thus, "whom" is correct.
Now, where do you think the gap is in "I think is responsible for the accident"? That's right. It's before "is": "I think [that] __ is responsible for the accident. Would you say,
*"[strike]
I think [that] him is responsible for the accident"[/strike]? No. You would use "he" (subjective case). Thus, "who" is correct.
The technique I have just outlined works for native speakers. Whether it will work for you will depend on whether you have developed your grammatical feeling for English enough to perceive the obviously ungrammatical sentences above as obviously ungrammatical. If you haven't, the procedure won't work for you.