The following are known Action Replay Codes for Wario Land 4 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
4959bbbd
b037b8de
d7bdf6ac
fbdf94c0

Infinite Health
b18da37c
7de6d952

Max Heart Gauge
538d2806
1eac5c17

Max Coins
d0918d13
7d06f45a

Have Keybird
e805c69e
c0aab49f

Have Piece 1
e0b67202
c82f3720

Have Piece 2
918a8e1e
bb1d5e5d

Have Piece 3
6173c6d1
7454e3bd

Have Piece 4
e708a358
4f01cb4f

Always Puffy Wario
e7d3ec42
3758dcb3

Always Bat Wario
8c02c876
365d6064

Always Flat Wario
caaadde6
bd3979cd

Always Bouncy Wario
f9eab670
580717d8

Always Zombie Wario
9af779fd
e75e7b5d

Always Snowman Wario
89ab08b8
4d4d4ac6

Always Flaming Wario
eb75e072
0dddfa1e

Always Fat Wario
b9a692d9
a43617b8

Always Frozen Wario
f740d2b3
180662a0

Have Disc 1
abc399da
97baf019

Have Disc 2
8e9497a3
56c4d5e0

Have Disc 3
e93e391e
6dcb03fd

Have Disc 4
a8c33d87
41fedf2e

Have Disc 5
2cb094fc
d82fcf7b

Have Disc 6
9f73a6df
b7c39423

Have Disc 7
c39f020e
7cd03cc1

Have Disc 8
08919871
3c7ea7a0

Have Disc 9
2981e72f
3f19a0c5

Have Disc 10
0f3814ad
8d0cb58b

Have Disc 11
66f55aeb
cf73f860

Have Disc 12
c1dcde95
a9359732

Have Disc 13
0fe765d3
1176c22b

Have Disc 14
c200eed7
941537b2

Have Disc 15
12ab547c
c0747046

Have Disc 16
8646dc53
197760cc

Press AR Button: Medals (Code 1)
ca588faf
93de1498
ccae0182
80c67098

Press AR Button: Medals (Code 2)
ca588faf
93de1498
c94706e8
f5feb0a8