Huh? That directly contradicts The Needs of the Many, where...I think to resurrect Data you do need to first build a new body. B-4 is important because he may have backup of Data's sensory input logs.
Just check out Christine Thompson's first short story in "Star Trek Magazine".
B-4 and Captain Data both exist as separate entities, so fans assuming that the Data personality permanently overran B-4, some time before "Countdown", were incorrect.
http://www.trekbbs.com/showthread.php?t=176575
Data's downloaded "Data Matrix" does overwrite B-4, although the situation and the way it plays out are far different to the assumptions of unjust murder mentioned above.