I need to measure the bandwidth of 4 specific sites. Is there a tool free available on the net that helps us achieve this?
It depends what you mean by 'bandwidth'.
If you want to measure the available network bandwidth then a lot is going to depend on where you measure it from. And you're only going to get an accurate measurement by filling up the bandwidth. You'll also need to learn a lot about TCP tuning to understand any measurements you get.
If you mean how much bandwidth each site is using then have a look at mrtg or argus. This pre-suposes that each site runs on a seperate IP adress. For different vhosts on the same address you'd have to use something capable of reconciling protocol level information with packet level information like pastmon. OTOH if you just want a rough figure based on the content size (i.e. ignoring header size and packet retransmits) any of the free webserver log analysers should fit your needs - I use Awstats.
If you want to monitor just the http/https traffic there is couple of easy solutions:
1) Create new log type for every different site (vhost) that contains only the date (in specific syntax you want) and the bytes from the transaction. After that you can easy parse that log file and put the data whereever you want. If you use apache here is an example for your vhost:
The output of that log should looks similar:
2) There are many log analyzers such as awstats or webalizer. They also make graphs and gives you extended information about the visitors.
3) Look for mod_cband (again for apache). It's very flexible and easy for use. It was a dead project, but the creators bring it back.