While applying to masters programs, how important is GPA of unrelated under-graduation? For example: I'll soon apply for masters in Statistical Machine Learning after 4 years of relevant work experience. I've done under-graduation in Electrical Engineering from a highly reputed Indian university, obtained poor grades however. 

I wanted to understand if admission committee emphasizes GPA in unrelated program when I have considerable work experience in field I'm applying to.
This can really only be answered by the faculty of the university you apply to. However, I would guess that your old grades will have a negative impact. The reason is that they were earned in a course of study and you are attempting to join that world again, not the world of work. 

The impact of your work experience will differ (greatly) from institution to institution, with some weighing it very heavily, but others less so. 

However, your record is what it is and people will see and judge it. You won't know what that judgement might be, however, until you make application and state your case for admission. People will want to know what is different now about your aptitude for and commitment to study and learning. You need to convince them that you will do better in the future than your record of the past indicates.