Mega Man Battle Network 4

Battle Network 4 differs from Battle Network 3 in many ways, the most notable of which is the concept of DoubleSoul (Soul Unison in the original). DoubleSoul is similar to how the classic-style Mega Man could acquire the weapons of the Robot Masters that he has defeated: by using DoubleSoul, Mega Man.EXE could use the powers of other NetNavis in the same manner as how classic Mega Man could use the powers of the Robot Masters through their weapons. To perform a DoubleSoul, which lasts for a temporary amount of time (three turns), however, a chip must be sacrificed for the battle.

Some Double Soul examples and their abilities are:

At a space research center known as NAXA, scientists discover a massive asteroid speeding toward the Earth that would cause the extinction of all life. After a call is put out to the world's greatest scientists, including the father of the main protagonists Yuichiro Hikari and a man known as Dr. Regal, they decide to direct the asteroid off-course with a laser. When that plan fails, Regal suggests searching for a world-class Net Operator and NetNavi.

Meanwhile, main protagonists Lan Hikari and MegaMan.EXE experience trouble both in and around their hometown of ACDC and on the Net. After fighting ShadeMan.EXE, he drops unknown data which is later revealed to be a DarkChip, a program described as unleashing great power at the cost of a Navi's "soul." The duo are warned to never use it under any circumstances.

This game is known to become unplayable on original Nintendo DS systems. If the player leaves, and then returns to, the map while in Park Area during WoodMan's scenario, the game will hang indefinitely. This incompatibility was officially acknowledged by Nintendo. It does not occur on any other Nintendo DS family system or on any Game Boy Advance family systems.

Capcom of Japan gathered together their surplus of Rock Man EXE 4: Tournament Red Sun and Rock Man EXE 4: Tournament Blue Moon games and put them in a special edition box that was released in Japan only on December 31, 2004, along with a special E-Card for the E-Reader.

The two versions of Mega Man Battle Network 4 entered Japanese sales charts as the second and fourth best-selling games of their release week and continued to appear on the top 10 best-sellers list for the following six weeks. The game sold 535,836 copies combined by the end of 2003 and an additional 393,014 copies by the end of 2004. Mega Man Battle Network 4 has sold 1.35 million copies worldwide as of December 31, 2008, becoming the best-selling game in the Battle Network series.