1) OK. Yes, you haven't decided for sure yet. 2) OK. Means the same as "I think his name was Bill", but shows more difficulty in remembering the name and less certainty that "Bill" is the correct name.
3) Possible as a somewhat humorous and sarcastic answer to "What are you doing?", said at a party, for example.
4) OK. Slightly more enthusiastic and subjective perha