Hi ,
Since it is dependent requirement , it would not be 'Lot size' . Lot sizing works on Receipt elements(PR/Plan Order) .
It must be Yield or Scrap% either in Material master/BOM/ Purchase-info record for vendor, if vendor is selected by MRP.
If only one item is proposed to be excess , it should be scrap% ... Yield would inflate qty for all 4 child items.
There is Operation scrap as well & vey good help in 'Net Id' field of BOM item on how scrap % are calculated( Various fields,
Regards,
Jatin