Hi
You can use Co product scenario for you requiremenut in which,you need to maintain co product in Material master for all 4 product and BOM for any one the as header material and other 3 as component with co product tick in item details.Also you need to maintain apportionment struture in MM for BOM header material with the help of CO consultant.
I hope you are not working with REM.
check this link:
http://help.sap.com/bp_dm605/BBLibrary/HTML/235_EN_DE.htm
Regards,
Anupam Sharma