Source document

/forum/service-tax-e-filing-of-return-170979.asp

If u are paying any Interest/arrears of Previous return/period , then only source document no is required to be filled in 4B . The format may be REGNUMBER_SP_YYYY_nnn_SCN  for showcause . It may be due to Demand Note, OIO,OIA ,etc. Whatever amount has been paid by GAR7 challans ,it needs be filled in  format prescribed .

Your STREGNO_PERIOD&YEAR_TYPE OF RETURN (pertain to which)

The source document is reference of document against which payment of service tax, interest or penalty is paid either in cash or in credit. The document could be audit report, order of any service tax authority or demand notice. The amount could be arrears or demands raised by authorities. This number will help field officers in knowing exact outstanding left against which order or report.
ST REG.NO._AC(Assist.Com. , respective authority)_period(e.g.,2011)_3digit order No._OIO

If sou moto like interest for late payment , then create a dummy to fulfill the format

ST Reg. No._AC_2011(year)_000_OIO

OIO MEANS ORDER IN ORIGINAL