The following are known Action Replay Codes for Mega Man Battle Network 4: Blue Moon on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
193565bf
4742d9f1
6d6ae025
23ca30ae

Infinite Health
a8e6982b
e6cc4aa0

Full Custom Gauge
9c5057e6
49ff8345

Chips Do 9,000 Damage
f1c68bd4
fecac79d
119b1541
da4a7313

Press Select: Deletes All
6d473537
693e0d4d
0738ece9
743be779

Infinite Zenny
a5cb26d1
bd0c4f71
89d31782
c3840839
f2e30e55
a90fdd3b
5fd329f8
bb39ea3b
b38a5a5a
d0db60bf
222c5768
3b0d60e6