I got US B1/B2 for 5 years, but did not use it for the first 4 years. Now I intend to use it. Do you think it will be an issue considering I did not use it for the original purpose (which was a particular business event) for a very long time and I want to use it for a different business purpose now. I am particularly concerned that I did not use it for long time.
Only a CBP officer can decide if you can enter the US. Although I cannot point you to an official source for this, but from my colleague's experiences, yes, you can present the valid visa to the CBP officers at the port of entry. I would assume flexibility in your visit was expected as the visa provided was for 5 years. You may have to provide additional details to the CBP officers that your intent of entering the US is for a genuine business/visiting purpose.
I found this site with a quick google : http://www.immihelp.com/visitor-visa/tourist-visa-faq.html The 5th question in there is similar to yours.