the {4} in your regex makes me think perhaps you wanted something like this to strip out the leading blocks of four which I assume go hex.

$text=~s/(^|\n)[0-9a-f]{4} /\1/g

# gives
636d6400 5589e583 ec0831c0 83e4f0e8 cmd.U.....1.....
00000000 e8000000 0083ec0c 68000000 ............h...
00e80000 000031c0 c9c39090 90909090 ......1.........
[download]

Cheers,
R.