To answer the first question, RB4 DLC is usually announced on Tuesdays at 10 AM EST and released Thursdays sometime between 12 AM EST and 10 AM EST, maybe late night wednesdays.
To answer your second question, I'm not really sure what you are asking, the wording of the question seems a bit off. What do you mean songs not on the list? If we're talking RB4 content, anything released from October 6th, 2015 to now is all RB4 content.