We have an image that shows Vol.1 which will include cassettes robots Ravage, Laserbeak, Eject and Rewind that will apparently make up the four pack. The Japanese name of the set "Casette Daisakusen Vol.1" was taken from the title of the Headmasters episode #4 cartoon.
According to the eHobby page, the #15 Encore Transformers Cassette Big Mission set of Four Vol.1 will be available in March 3009 for 4,200 JPY (~$46 USD).
Note: If you didn't catch the "Vol. 1," it likely refers to the first in a series of Cassette packs that will be released later in the Encore series. Knowing Takara, probably with themes as well.