If you want to monitor the bandwidth usage for all the four laptops separately then by monitoring the router interface you cannot achieve that. By monitoring the router interface you will get the overall bandwidth details of all the four laptops.
To find the bandwidth usage of each laptop and every laptop individually. Install the SNMP service in the Laptop (if it is windows go to add or remove program --> Add or remove windows components --> select Management and monitoring tools and install the SNMP service
Give the SNMP community string and now you can monitory the bandwidth usage of all the four laptops individually by using any of the free tools like MRTG etc.
All operating system will support SNMP
The MRTG monitoring tool you have both in windows and Linux.