"Ponderous" means dull and laborious. It is fine to describe a book or a speech as "ponderous". Seriousness of subject matter alone does not make a book or movie ponderous. The treatment of the subject matter makes the book or the movie ponderous. So 1 and 3 are safe. For 2 and 4, we don't know. It depends on the writing style. A speech or book on a very serious subject, such as marriage, can be interesting and entertaining. A book on an interesting subject, like sex, can be dull and laborious, and therefore ponderous. Your definition is from the Cambridge Learners' Dictionary. It tries to be simple, so that learners may find it easier to understand the general idea. For more precise and sophisticated understanding, the Oxford Dictionary comes to the rescue. Remember "dull and laborious" (Oxford).

Ponderous means that something is very heavy. When used to describe a book, or a speech, it is not used to say that it is boring (though it may be), but rather that the subject matter is very serious. So, the answer to #1 and #3 is No, you cannot call a book or speech ponderous because it is long and boring. The answer to #2 and #4 is Yes, you can call a book or speech ponderous if the subject matter is very serious.