use yourtablename
replace all yourfieldname with update_sku(yourfieldname)
use

function update_sku(cString)
   if type('oRegExp') = 'U'
      oRegExp = new OleAutoClient("VBScript.RegExp")
      oRegExp.global  := true
      oRegExp.ignoreCase := true 
      oRegExp.Pattern := "(\D+)(\d+)(\D+)"
   endif
   a = oRegExp.execute( cString )
   cNew = a.item(0).submatches.item(0)
   cNew+= iif(a.item(0).submatches.item(1).length = 4,'0','')
   cNew+= a.item(0).submatches.item(1)
   cNew+= a.item(0).submatches.item(2)
   return cNew