1. If you really have to choose, choose "that", but "which" is used as well. 2. Which 3. Which 4. Same as #1.
In general, you have to use "which" when it's in a non-restrictive clause, which comes after a comma and is the kind of clause you could put in parentheses if you wanted to. In the other cases, you can use both, but "that" is considered better by some and it also