@rohitpahwa
One of the things they will be doing is cross-reference the "duties/responsibilities" listed on your letter of reference, with the NOC-code that you entered. If that information is significantly mismatching, it's possible for them to reject that work experience. The best ways of doing this:
* One letter of reference, containing all positions you held, and for each of them seperately a list of duties/responsibilities
* Four different letter of references, one for each position
I would discourage one letter of reference with a list of duties/responsibilities that cover everything, as it creates confusion and it creates extra work for the VO to match everything up. If two or more positions you held have the same NOC-code, you can probably just merge it together. In either case, you should have it listed when you started each position and where you were based (India/US)