Mega Man Battle Network 4 is the fourth game in the Mega Man Battle Network series. Like Battle Network 3 before it, Battle Network 4 has two different versions, Red Sun (Tournament Red Sun in Japan) and Blue Moon (Tournament Blue Moon in Japan), that differ in story and gameplay details. Battle Network 4 is compatible with the e-Reader, and can be linked up with Rockman EXE 4.5 Real Operation and the Battle Chip Gate for NetBattles. It is able to link with Mega Man Zero 3. (wikipedia.org)