The first three are perfect, but based on the opening clause of the fourth example, it should also be, "Who knows what he would have done if I hadn't been there." If you wish to use "if I wasn't here," then you would begin it with "Who knows what he would do (if I wasn't here)." The first part is conditional and therefore takes the subjunctive, but the second part is indicative, saying that it is known to be true and unquestioned that you are there. If it is uncertain that you will be there, the sentence would read, "Who knows what he would do if I weren't here."

Although they are rhetorical (not requiring an answer), all three of these sentences are actually questions and should be punctuated with question marks rather than periods. "Who knows what he might have done if I hadn't been there?" etc.