You can vote for which of these four (each of the two Battle Network variants is counted as one game for release date purposes) you want to see launch on May 7 over on the Capcom Unity blog. The remaining games will then be released chronologically, one per Thursday, for the remainder of May. Unfortunately, we can't see the current results of the poll, so the winner will be a mystery until after the end of the voting period, which runs from now through April 29.
Among GBA games on the Wii U's Virtual Console, the Mega Man series is better represented than any third-party franchise, with four games already available, including the first two Battle Network games. A price for the new games wasn't announced, but the previous Mega Man GBA games cost $7-$8 each.