Yet another way to do it. Somewhat slower and less arcane than kaif's, but maybe adaptable to other situations. And it saves a few keystrokes. :-)

# Reused your sub name
sub test4 {
  return unpack("%4b*", pack('C', $_[0] & $_[1]));
}
__END__
          Rate Test5 Test4 Test0
Test5  88345/s    --  -34%  -47%
Test4 133240/s   51%    --  -20%
Test0 167395/s   89%   26%    --
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