Just a start:

perl -wne '
  $h{$1}++ if /bn(\d{4}),/ }{$,="\t"; print %h, "\n"
' datafile
[download]

To see the actual code, use:

perl -MO=Deparse -wne '
  $h{$1}++ if /bn(\d{4}),/ }{$,="\t"; print %h, "\n"
'
[download]

-- 
Ruud